Thomas Tuchel's disappointment was clear to see when he revealed the extent of the injury issue that has derailed Mateo Kovacic's season. Speaking in a press conference, he said he 'missed' the star and admitted his setback was 'really sad'.
An unsung hero for Chelsea since joining from Real Madrid, Kovacic has gone from strength to strength this year. It is no mean feat to escape from the shadow of N'Golo Kante and Jorginho, but he has done just that to link up play superbly.
His dedication on the training field - which in a cruel twist of his fate likely caused his hamstring problem - and ability to wrestle back control in midfield has helped him to become a crucial cog in the club's relentless winning machine.
'You can call Mateo at three o'clock at night and call him for tactical training, for defensive training or set piece training or just call him,' Tuchel said.
'He will be there with a smile and do exactly what he's needed to do. He is such a fantastic guy. Such a fantastic team player.
'I was a huge fan of him when he played for Real Madrid. I told him straight away I remember El Clasicos I watched with him.
'I could feel there is such a big potential when I met him and this friendly, humble guy with such a big heart for football and such a passion for training.
'It is a pure joy to be his coach. We encourage him to trust his potential, to simplify his game a little bit and not look in every scenario for a third and fourth solution because sometimes he already has his first and second solution that are pretty good.
'He always thinks about the others. He would always pass the ball on the goal line if there was somebody to score and not him.'
This selfless nature has shone through on several occasions already this campaign. During his lay-off, Chelsea's form has not taken a dip but there is no doubting that the team have been missing his silky passing, energy and ball carrying.
So, how do Opta's statistics look? When comparing the numbers for when Kovacic starts for Chelsea and when he doesn't, there is a clear difference.
He has started eight top-flight games this season, and missed out on five. Out of those eight, Chelsea have won seven, but this takes a substantial drop down to two when he doesn't start. They have also drawn three of the games in his absence.
There is also a vast difference between the amount of goals the team scores with and without their midfielder. Whilst he was in the team, Chelsea scored a hefty 22 goals - but when he was sidelined through injury, they scored nine.
As a result, their average goals scored is 2.8, compared to 1.8 without Kovacic. They are tighter at the back with him, too. Their average goals against stands at 0.3, but this figure is 0.6 when he doesn't line up at the heart of their team.
Perhaps the most intriguing statistic is Chelsea's win percentage. At 87.5 per cent with him in the side, Kovacic's influence over their results appears clear. Without him, it is 40 per cent, a significant collapse in fortunes in the top-flight this term.
Finally, their points per game is 2.6 with Kovacic and without him, it is 1.8.
It is also worth weighing up Kovacic's statistics against his team-mates. When looking at goals and assists combined this season, he ranks very highly indeed. In fact, only Reece James - the marauding, in-form wing back - has bettered him.
James, from his 10 games and 638 minutes played, has a remarkable four goals and four assists. In one game less, but more minutes, Kovacic has scored one goal and picked up five assists, taking his combined total to an impressive six.
Below him, Mason Mount, Ben Chilwell, Romelu Lukaku and Jorginho also feature on the list. The most notable comparison is between the output from this year's third-placed finisher in the Ballon d'Or nominations and his partner, Kovacic.
Jorginho has played 12 games this campaign, totalling 906 minutes. From those appearances, he has scored two goals and registered one assist - and has, in the process, further underlined how underrated Kovacic currently is.
Furthermore, he is ranked fifth in the squad for ball carries, and even sits top for shot-creating actions. These numbers reveal the extent of his playmaking abilities, but it has also been revealed that the defensive aspects of his game are excellent.
That is because, according to FBref, he is in joint first for tackles, alongside Jorginho, and is second for putting pressure on opponents. Tuchel has afforded Kovacic his fair share of defensive responsibility, but this has been a task he has excelled in.
It hasn't always been smooth sailing for him at Stamford Bridge, though. At the start of the 2018-19 season, Kovacic arrived through the door on loan, just a few weeks after Jorginho was also recruited to bolster Maurizio Sarri's midfield options.
Despite his initial jitters, Chelsea made Kovacic's move permanent the following summer, and he has continued to blossom with every passing season since then, winning the club's player of the year award at the end of the 2019-20 season. Yet he was arguably enjoying his best football before his injury struck.
One of the club's most important players during their Champions League-winning campaign last term, Kovacic has transformed himself into a driven and relentless metronome at the heart of the team.
The timing is perfect, too. Kante, despite his age-defying work-rate, is struggling to remain healthy across an entire season and Jorginho's minutes must be carefully managed over the coming months to ensure he avoids burning out.
For most clubs, losing two classy midfielders would prove an issue, but Chelsea are blessed with a third in Kovacic. Tuchel, therefore, is able to rotate in the centre of the pitch, and Kovacic will be as adept as ever at keeping play ticking over.
His slightly-tweaked role this year has undoubtedly contributed to his upturn in fortunes. The 27-year-old has had more freedom than ever to push forward and make incisive, line-breaking passes, but is also trusted to break up attacks.
The recruitment of Lukaku has benefitted Kovacic as well. A classic No 9 capable of holding up the ball and bouncing off his creative team-mates, Lukaku has formed a good partnership with Kovacic and often been able to play off him.
With these two added dimensions to his game, it is little wonder he is one of Chelsea's best players this season. And, in another hat-tip to the seemingly flawless Tuchel, Kovacic has not suffered defensively despite the added burden.
Press-resistant and a formidable distributer of the ball, he has been reliable when Chelsea are placed under pressure. The fact he is also able to dribble his way out of danger and spark counter-attacks only further highlights his importance to the team.
Promisingly, there is still room for improvement. There are creases in his game which, if ironed out this season, will help the four-time Champions League winner cement his place as one of the best box-to-box midfielders in the world today.
Out of Kovacic's rich highlight reel from this campaign, it is perhaps his pass for Lukaku's opening goal against Aston Villa which stands out from the rest. It looked stunning in real-time, but its absolute precision was laid bare in slow motion.
The frontman darted in behind the backline early, and Kovacic managed to slide the ball beyond Axel Tuanzebe to find his team-mate. It was a pass of jaw-dropping simplicity, and was achieved while the midfielder was on the move and glancing up.
Having now played for Inter Milan, Real Madrid and Chelsea, his quality is not in question. What has hampered him in west London is the more eye-catching work from those around him, but he is now standing out more every week.
His acclaim is richly deserved and, despite Tuchel's winning machine remaining on track in his absence, Kovacic will only breathe more life into their title challenge when he returns.
Sadly, that will not be against West Ham this Saturday.