The injury suffered by Liverpool's Adam Lallana "looked really serious" to Jurgen Klopp, but the Liverpool manager also insisted it is "too early'' to judge whether it will jeopardise his World Cup hopes.
Lallana limped off the pitch just five minutes after joining the fray as a second-half substitute in Liverpool's 2-1 Premier League win at Crystal Palace.
Mohamed Salah struck his 29th league goal of a remarkable season to add to Sadio Mane's effort, after Luka Milivojevic's penalty had handed Palace the early lead.
Lallana picked up a possible hamstring injury and left Selhurst Park on crutches, in another blow to his bid to make a lasting recovery from his long-term thigh problem.
Asked if Lallana's latest setback could harm his World Cup chances with England, Klopp replied: "It's absolutely too early [to tell]; it wouldn't be nice if you ask that so early after consequences.
"It's absolutely bad, but we don't know exactly what it is yet. In the first moments I saw him it looked really serious to be honest. It looks like something with the muscle.
"He's our friend, our boy, our mate, our player, having a difficult season already.
"Maybe one of the biggest achievements of that team is that they could play a season so far like they have played without Adam Lallana. I never would have thought it's possible.
"And so we are all desperately waiting for him, and now coming step by step back, and now this -- it's a big, big blow for us.''