Manchester City manager Manuel Pellegrini expects skipper Vincent Kompany to be out of action for at least four weeks with a groin injury.
The Belgian international was forced to limp off during City's emphatic 4-0 win over Newcastle United on Monday evening with a knock that his manager described as a "serious injury" following the match.
"I prefer the doctor has the final word on this but I think he will be out for at least one month," Pellegrini said.
"If we do strengthen (at centre back) it will not just be because of Vincent Kompany's injury. We have a very good squad and we see what happens.
"Today or tomorrow we will know exactly how long Vincent will be out for."
Kompany's injury could possibly see him ruled out for the first Manchester derby of the season on September 22, but his manager will have more immediate concerns given the dearth of options he has at centre half.
Matija Nastasic returned to training earlier this week but has not played a game since July when he suffered an ankle injury during his club's Barclays Asia Trophy campaign, an injury that was initially predicted to rule him out for six weeks.
Micah Richards is also waiting a return date after picking up a hamstring injury before the visit of Newcastle, with City expecting to be without the 25-year-old for at least two weeks.
Having allowed Kolo Toure to leave the club on a free transfer during the summer, Joleon Lescott is club's only established option at centre half fully prepared for Sunday's trip to Cardiff City, a development which will likely see the club attempt to bring in another centre half as soon as possible.
The Premier League runners up are keen to recruit Martin Demichelis, who enjoyed an impressive campaign playing under Pellegrini at Malaga last season.
The Argentine centre half left the Andalusican club in the summer to join Atletico Madrid, but it hasn't deterred the Premier League club from trying to strike a deal for the 32-year-old. Real Madrid defender Pepe has also been linked with a move to the Etihad Stadium.
Ahead of Sunday's testing trip to Cardiff, Pellegrini may have to look to 22-year-old Dedryck Boyata to step into his side.
City are likely to be without their captain for games against Cardiff, Hull City, Stoke City and that pivotal encounter against United at the Etihad.