Manuel Pellegrini challenged his Manchester City centre-backs to cope with the loss of captain Vincent Kompany for a pivotal spell in their season.
Kompany, who had missed City's previous eight games, limped off nine minutes into his comeback in Saturday's 4-1 win over Sunderland with another calf injury.
He underwent a scan on Sunday and City could be without the defender for Tuesday's trip to leaders Leicester, both legs of their Capital One Cup semifinal against Everton, the start of their FA Cup campaign and several key Premier League games.
"He's very important because first of all he is captain," said Pellegrini. "He is captain because has a lot of important things to give -- his performance, his personality. We have to play so many games in January that it will be important for him to have fully recovered."
City spent £31.5 million on Nicolas Otamendi in the summer and bought £42 million centre-back Eliaquim Mangala last year and they have been the regular central-defensive duo in Kompany's absence this season, while the veteran Martin Demichelis offers another option.
"Without Vincent we must try to continue being competitive," said Pellegrini. "That depends on the performances of Mangala, Otamendi and Demichelis."
In the 17 games that Kompany didn't start for City this season, the team have yet to keep a clean sheet. But Pellegrini added: "We cannot think that all the mistakes are just because Vincent isn't playing.
"He's very important. That doesn't mean that we have to think that with Vincent we're not going to make any mistakes, and without him we cannot win. It's an important challenge for the other players."
Pellegrini drew confidence from City's efforts at the end of last season, when Demichelis and Mangala were paired.
"This team is better with Vincent than without him," he said. "But if you review how many games we played over the last two seasons I've had, I think Vincent plays a maximum 70 percent of the season. And we won titles, and the last six games [of last season] without him."