Virgil van Dijk was withdrawn while Liverpool were 2-0 down at half-time at Brentford on Monday - with fears it could have been due to an injury.
It was a shambolic first-half showing from the Reds, which saw them concede from an Ibrahima Konate own goal and then Yoane Wissa's strike for the Bees.
The London club ended up as 3-1 winners over their Merseyside rivals as both sides played their first Premier League action of 2023.
Klopp was not impressed with the first 45 minutes, and made a trio of substitutions, including taking off 31-year-old captain Van Dijk.
Harvey Elliott, who was booked in the first 10 minutes, and Konstantinos Tsimikas were the other two players taken off at half-time.
Speaking on Sky Sports during the interval, Jamie Carragher said: 'Could have been anyone. It could have been five. He had to make changes because that was an unacceptable 45 minutes.'
On the Liverpool skipper, Carragher added: 'It will be interesting to see if that’s an injury or if he has just came off because he has struggled.'
It may have been that Van Dijk was taken off because of injury, with his right leg strapped up on the bench as the rest of the match played out.
Klopp's second-half additions were Naby Keita, Joel Matip, and Andrew Robertson - who took on the armband for the remainder of the match.
The manager's actions seemed to work in Liverpool's favour, as they clawed one back in the 50th minute through Alex Oxlade-Chamberlain.
However, Bryan Mbeumo scored with six minutes to play, regaining Brentford's two-goal cushion after the pair of first-half goals.
The Bees also had the ball in the net an additional two times in the first half, but those attempts were ruled out for offside.
Liverpool's defeat left them in sixth place in the Premier League, and are now just two points clear of Brentford, who rose to seventh.
Former Southampton centre-back van Dijk has made 145 appearances for Liverpool since joining them in 2018, and featured in the heart of the Dutch defence at the 2022 World Cup.