The Spanish striker was taken to a nearby medical facility after picking up the knock in Saturday's controversial draw with Everton at Stamford Bridge.
Diego Costa, who had earlier scored, was taken off with 10 minutes of normal time remaining, hobbling off with a shin injury. Medics are baffled as to how he actually got injured.
"We don't know the result yet and I hope it's not bad," Chelsea interim manager Guus Hiddink admitted. "He was in a lot of pain, that's true."
Hiddink, who took over from José Mourinho last month, has made it clear he is not in any hurry to sign another striker this month in case of a lengthy layoff.
When it comes to Costa, anything is possible, the former Atletico Madrid forward was hospitalised with a rib injury only three months ago, also forcing him out of the game.
"We hope Diego will come out [of hospital] in good health," Hiddink added.
Chelsea can't really afford any more misfortune. Saturday's draw means the club is ever closer to relegation than the previously hoped for top four finish.
Yet many critics claim the club shouldn't have been awarded a draw at all!
Chelsea captain John Terry netted a 98th-minute equaliser nearly a full minute after the allotted stoppage time had elapsed and from an offside position.
Everton manager Roberto Martinez was furious: "It's a clear-cut offside that should have been spotted. To make it worse, it's 50 seconds over time. It's something that's not good enough!"
Centre-back Terry had somewhat of a difficult game: not only did he score late and offside but he also put through an own-goal.
Hiddink admitted that Martinez had been correct to complain about the offside call.
"I can say now he is quite right," said the Dutchman.