Barcelona will be without star defender Dani Alves for up to a month after he injured his thigh against Rubin Kazan.
The Brazilian right-back pulled up deep into injury-time in their shock UEFA Champions League defeat on Tuesday.
Tests have shown the former Sevilla ace has ruptured a muscle in his right thigh, meaning he could miss up to six matches for the Spanish giants.
"This is bad news," stated head coach Pep Guardiola. "Dani is a man, not a machine.
"Now we must ensure he gets the best possible treatment because he's a key player in our team."
Meanwhile, Guardiola remained philosophical when quizzed about his side's surprise European defeat.
Barca top Group F, but only just and Guardiola insists despite the statistics, football sometimes cannot be explained.
"Football is like that," he added. "With the statistics that we had, in any other sport we would have won, but not in football.
"I honestly think that we had a good game, but results do not always show that.
"We've had a good game, but we've lost, and you cannot do anything about that. It's disappointing because we could have taken an important step towards the last 16."
Barca's Xavi put the result down to bad luck as his team-mates dominated possession and had 13 shots on target while also hitting the woodwork twice.
"It was just absolutely rotten luck and the way the match went we did not deserve to lose," said Xavi.
"Rubin had two attacks and nothing more and have taken away a prize they did not deserve," the Spain international added.
"We played well but it makes it hard when your opponents only come to defend."