Eden Hazard could be sidelined for two weeks after injuring his calf during Chelsea's 2-0 Champions League win over Paris Saint-Germain on Tuesday, manager Jose Mourinho said.
"With a muscular problem, even when it's not serious, you cannot play normally for a couple of weeks minimum," Mourinho told a press conference.
"But I have no idea of the dimension."
Hazard came off after only 18 minutes of the quarter-final second leg at Stamford Bridge and headed straight down the tunnel.
The Belgian winger's replacement, Andre Schuerrle, scored the opening goal as Chelsea stormed back to secure a 3-3 aggregate draw that sent them into the semi-finals on away goals.
"He comes out after 10 minutes saying he couldn't do it," Mourinho said.
"He has some problem. It's a calf. It's a muscular problem."
Hazard, 23, has been Chelsea's stand-out performer this season, having scored 17 goals in 46 appearances.
A period of unavailability of two weeks would leave him at risk of missing the first leg of Chelsea's Champions League semi-final, as well as their trip to Premier League title rivals Liverpool on April 27.