Manchester City fear Vincent Kompany could be out for a month after he suffered another muscle injury as the club progressed in the Champions League.
The City captain, who has been plagued by recurring calf problems, was forced off after just seven minutes of the goalless last-16 second-leg tie against Dynamo Kiev on Tuesday night.
The Belgium international centre-back was injured as he slipped in a challenge with Kiev midfielder Vitaliy Buyalskiy and immediately kicked the ball out of play before signalling to the bench and hobbling down the tunnel, with a look of profound disappointment on his face.
Kompany returned last month after suffering a grade-two tear to his calf against Sunderland on Boxing Day and, after City drew 0-0 at the Etihad to progress to the quarter-finals 3-1 on aggregate, boss Manuel Pellegrini confirmed Kompany has injured the same muscle again.
"Vincent, unfortunately, has a muscle injury in his calf like he had before," he said. "We'll know more tomorrow or maybe the day after tomorrow how important it is, but always a muscle injury is minimum of one month."
Fellow centre-back Nicolas Otamendi was also forced off with a problem, but Pellegrini does not think it is too serious.
He said: "Otamendi has an important kick but we think just a couple of days for him to recover. We will see the way he develops."
Kompany's injury is a huge blow to City's dwindling title challenge. He could miss crucial matches against West Brom and Bournemouth, as well as the clash against United at Old Trafford, which is live on Sky Sports 1 HD from 3.30pm.
The centre-back's lay-off at the start of 2016 was the 12th time Kompany has been sidelined due to a calf injury since the start of the title-winning 2011-12 campaign, and his third this season.
City manager Manuel Pellegrini, who at the time voiced fears Kompany's injury could be career threatening, warned last month that the 29-year-old could break down again.
Eliaquim Mangala replaced Kompany against Kiev and Yaya Toure took over as captain, but less than 20 minutes later City suffered a further defensive blow when Otamendi was forced off with an apparent knee injury.
City now head into Sunday's Manchester derby with both of their first-choice centre-backs injured.
Watch City take on Manchester rivals United live on Sky Sports 1 HD on Sunday from 3.30pm.