Manchester City captain Vincent Kompany is likely to be sidelined for about a month after suffering a recurrence of a calf problem on Tuesday as his side qualified for the Champions League quarter-finals.
Kompany was forced off in the fifth minute but City went on to reach the last eight of the competition for the first time after drawing 0-0 at home to Dynamo Kiev to advance 3-1 on aggregate.
First-half injuries to captain Kompany as well as Nicholas Otamendi cast a pall over the evening at a hushed Etihad Stadium, leaving City vulnerable in defence ahead of Sunday's derby with Manchester United in the Premier League.
"Vincent again has a muscle injury in his calf," manager Manuel Pellegrini told reporters.
"We'll know tomorrow, or maybe the day after tomorrow, how bad it is but always a muscle injury is a minimum of one month out."
Belgium central defender Kompany, who only returned from a calf problem in February having been out since December, will have to miss Sunday's home Premier League derby with neighbours Manchester United.
"This is difficult for him because he came back very well and played six games in a row," added Pellegrini.
City kept it tight on Tuesday night, refusing to gamble with their healthy 3-1 first-leg victory.
"We didn't take any risks," said Pellegrini. "For our team it was important to keep the ball and not allow them to create chances.
"They knew before the game they had to score three goals and that's not easy. We didn't have too many chances because we didn't force the game.
"We normally are a team that always scores. We've received a lot of criticism that we never keep clean sheets but we've kept a clean sheet for the last three games," said Pellegrini.
"Otamendi has an important kick and can hopefully recover in the next couple of days."
While progress in the Champions League represents a historic achievement for Pellegrini, who will hand over to Pep Guardiola at the season's end, the injuries could have a costly impact on City's attempt to resurrect their domestic title challenge.
The League Cup winners have fallen 12 points below leaders Leicester City, although they retain a game in hand, and face challenges from both United and in-form West Ham United for their top-four place.
Pellegrini welcomed Yaya Toure back from the heel problem that had caused him to miss Saturday's frustrating 0-0 draw at Norwich City, but within seven minutes he was counting the cost of another injury.
After misjudging a bounce on the City right, Kompany slipped as he tried to recover the situation and immediately realised that he had done himself damage, passing the ball out of play and signalling to the bench before limping off with a look of disgust etched on his face.
The Belgium international, who has suffered 14 calf injuries since 2008, was soon joined in the home changing room by Otamendi, who came off second-best in a collision with Vitaliy Buyalskiy.
With inputs from agencies