Chelsea's Hakim Ziyech was substituted early against Leeds after sustaining an injury.
The Morocco international limped off 30 minutes into the Premier League encounter at Stamford Bridge and was replaced by Christian Pulisic.
The 27-year-old went down after passing to Reece James in the build-up to the Blues' equalising goal, with the defender overlapping on the right side to receive the ball from Ziyech and crossing in to Olivier Giroud to fire home.
Chelsea manager Frank Lampard confirmed after the match, which Chelsea won 3-1, that Ziyech suffered a hamstring injury with a full assessment to come.
"Hamstring, he felt it go, so we'll find out to what degree over the next few days," Lampard told Sky Sports post-match.
He has been a key player for Frank Lampard's team since his arrival from Ajax in the summer transfer window.
The attacking midfielder and winger has one goal and three assists in the Premier League this season as well as a goal in the Champions League.
He has been tipped by former Chelsea midfielder Cesc Fabregas to turn the team into one worthy of winning the Premier League.
“He's a very good player. He has this vision that you need in a top team like Chelsea, who want to win and be champions," Fabregas said.
“You could see that they were lacking ideas last season and maybe at the beginning of this season, but with Ziyech, you can see that he has this last pass.
“It's not only that he sees it, it's that he makes it right, at the right time. That's the decision-making that we talk about nowadays."
Chelsea went behind just four minutes into the home clash when Kalvin Phillips played it through for Patrick Bamford, who then rounded goalkeeper Edouard Mendy and knocked into the net.
Lampard's team were third in the table heading into the match, just two points behind the top two teams Tottenham and Liverpool.
The Blues approached the game off the back of a 4-0 trouncing of Sevilla in the Champions League in midweek. Giroud netted all four of his side's goals as they dominated in Spain and sealed the top spot in their group with one game remaining.
After the Leeds game, the London side will be back in European action on Tuesday with a home clash against Krasnodar.
