Of course, concern may be overstating things when your backup is Petr Cech, but according to Simon Johnson of the Evening Standard, Thibaut Courtois has a hip injury of unknown severity that may keep him out of tomorrow's game at Leicester.
Standard Sport understands that first choice keeper Thibaut Courtois has handed the club an injury scare after suffering a hip problem during the goalless draw against Arsenal.
-source: Evening Standard
Presumably this happened towards the end of the match when the Chelsea goalkeeper landed awkwardly after getting fouled (or at least blocked) by a couple Arsenal players. Courtois recieved treatment on the pitch before finishing out the game.
Should Cech start, it would be his first appearance since the 2-0 win against Spurs in the League Cup final. Courtois had started all eight games since, and has kept three clean sheets on the bounce. Speaking of which, Cech is five short of the Premier League clean sheets record. He's unlikely to catch or break that this season. (Or ever...)