Manchester United stars arrived at the Lowry Hotel on the eve of the Champions League showdown with Barcelona, minus midfield duo Ander Herrera and Nemanja Matic.
The pair have been first choice for much of Ole Gunnar Solskjaer's reign so far, operating alongside Paul Pogba in his preferred central trio.
The more defensively-minded pair allow the World Cup winner greater attacking licence.
But the Norwegian admitted on Tuesday, as he ran through his list of injured players, that the participation of both against the Spanish champions was in question.
"We still hope maybe that Nemanja [Matic] can be ready," Solskjaer said. "Ander [Herrera] probably out."
As such, it looks increasingly likely that Scott McTominay and Fred will be called upon against Ernesto Valverde's men.
That pair played alongside Pogba in last week's loss to Wolves at Molineux.
Academy standout James Garner could also be set for a spot on the bench, as he was one of 20 players present.
The 18-year-old has already been the part of two United squads in Europe this season, in the last two away games, against Valencia and PSG.
Marcus Rashford was in attendance, with Solskjaer hoping he will be fit to lead the line.
"Marcus (Rashford) needed the extra warm-up and hopefully he won't get a reaction, he joined parts of it at the end but he couldn't join the boxes, which is the fun part of training, but let's see tomorrow.
"Hopefully he'll be ready."
One Barcelona star more than familiar with Old Trafford is Gerard Pique, who enjoyed a spell at United after joining from Barca at 17.
Speaking about his return, Pique said: "It's something lovely because the fans are fantastic," the defender said ahead of stepping out at Old Trafford for the first time since leaving in 2008.
"It's the same if it's Champions League or a league game or playing against a fourth division side - the fans are always cheering if you lose or you win.
"It's a different culture. You can see that both on and off the pitch. The fans are into their team.
"I've always said that the English culture, I think we could learn a lot from them and one of the things would be this instance."
United stars in attendance at the Lowry
Goalkeepers: De Gea, Romero, Grant;
Defenders: Lindelof, Dalot, Jones, Rojo, Smalling, Young, Shaw
Midfielders: Pogba, Pereira, Fred, Mata, Garner, Lingard, McTominay
Forwards: Lukaku, Martial, Rashford.