Joel Matip's injury during Liverpool's 3-1 loss to Borussia Dortmund on Sunday in North Carolina was the "most negative thing about the day" in Jurgen Klopp's eyes.
The defender limped off 22 minutes into the pre-season friendly, three games after returning to play following surgery on an injury that cut short his 2017/18 season.
Klopp said the source of pain on Sunday was a different complaint that appeared to be a small muscle tear, although the extent of any damage had not been confirmed.
"It's not the same injury. It's another muscle - it looks like, hopefully, only a little tear," the manager told liverpoolfc.com at a post-match press conference.
"We will see, but we cannot say anymore about it now. It is of course the most negative thing about the day. It's not cool. Things like this can happen, but you don't want them to.
"Maybe tomorrow or the day after tomorrow we will know more, but in this moment we only know that it was too painful for him to carry on."
Matip's last appearance of 2017/18 came in March, with the Cameroon centre-back sustaining a thigh injury against Crystal Palace and subsequently undergoing an operation.
He made his return to action in Liverpool's 0-0 friendly against Bury on July 14 and started again against Blackburn Rovers, which the Reds won 2-0.
Liverpool took the lead three minutes after Matip was replaced by Joseph Gomez during the International Champions Cup tie at the Bank of America Stadium in Charlotte, North Carolina, on Sunday.
Virgil van Dijk pulled off a magnificent header from the edge of the six-yard box, supplied by Andrew Robertson following a short corner by Rafael Camacho.
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Dortmund levelled up in the 66th, Christian Pulisic scoring from a penalty conceded by James Milner with a foul.
The sides looked to be heading for penalties until the 89th minute when Pulisic struck again following a counterattack.
Three minutes into injury time and the Bundesliga side snatched their third, this time off the right foot of Jacob Bruun Larsen at point-blank range.
"On a day like this, you have to stay strong and show desire still or kind of greed that you at least want to keep the result at 1-1," Klopp said.
"We have to learn from it, that's how it is, and then we have to go again."
But the collapse can only have added to any doubts embattled keeper Loris Karius may have had about the future, especially in light of Liverpool's world record purchase of Roma stopper Alisson last week.
In a shot at critics, Karius posted a message via an Instagram story after the game that read: "To those who take joy in seeing other people fail or suffer, I feel for you.
"Whatever it is that's happening in your life to hold this much anger and hate, I pray that it passes and good things come to you."
Liverpool's next pre-season friendly is against Manchester City at the Metlife Stadium in New Jersey on Wednesday.