Chelsea manager Guus Hiddink concedes that the shocking drop in the form of Eden Hazard is a cause for serious concern.
Although Hazard has been dealing with some injury problems, his poor season is still baffling.
Last year's PFA Player of the Year has managed only two goals in 38 appearances.
"It's a long period of absence for him. It was a bit alarming but now he is coming back. He practiced in three sessions of us," Hiddink told SkySports.
"But I told him now he must start performing. It's difficult when one has been player of the year and then has this fall. Some of it is due to injuries, and not in top form, it must be alarming for a player.
"It was breaking my head too; what can I do to help him get back on track? He fell too much into injuries and difficulties to come back and get out of there."
Hiddink also confirmed that striker Diego Costa could feature against Manchester City on Saturday, having served his three match ban.
"Diego Costa was working very hard. Normally when a player has been out that long they like to take a little break but he didn't. Also, individually, when we were on the road, he was here at Cobham working with the staff. He was eager to play."