Mauricio Pochettino has revealed that Christopher Nkunku has injured his hamstring and could be out for a month.
The French came to the Blues in the summer, but since then he has only made ten appearances in all competitions.
Nkunku suffered a knee injury in pre-season and made his competitive debut in December before suffering a hip injury.
He played in the Carabao Cup final defeat against Liverpool but missed the 3-2 FA Cup win against Leeds on Wednesday night.
In the latest from Chelsea injury updateThe 26-year-old is reportedly “in a rehabilitation program.”
And speaking at his press conference ahead of his trip to Brentford, boss Pochettino He said: “He suffered a hamstring problem and then we will see in a few weeks if he is available.
“Now he is recovering from Sunday. Bad luck, bad luck because suffering another injury again is hard for a player who did not play much during the season and who also came from the Bundesliga, from Germany, with some problems.”
Before Leeds' victory, Pochettino said: “We need to see him and evaluate him every day, but he will be out for three or four weeks. We don't expect any more.”
“It's difficult for him. We watched him when he arrived in preseason. He was flying in training camp until he got injured. Now almost eight months have passed and he's back involved, but he's not the same player as before.
“We have in mind the incredible Nkunku from pre-season, but he is not the same player due to the circumstances. That is why sometimes it is unfair when we judge.”
Nkunku is one of seven players on Chelsea's injury report, although Thiago Silva and Marc Cucurella are new omissions.
However, stars such as Reece James, Romeo Lavia and Benoit Badiashile are listed as “continuing in their rehabilitation programme”.
Nkunku came off the bench in Chelsea's Carabao Cup quarter-final victory against Newcastle and scored in the penalty shoot-out on his competitive debut.
His Premier League debut came in a 2-1 defeat to Wolves on Christmas Eve, with the striker scoring the final goal of honour.
The former Paris Saint-Germain and RB Leipzig player's second goal for the club was scored from the bench in the 4-1 loss to Liverpool.
Nkunku has only made two starts in the league this season, in the win over Crystal Palace and the home defeat against Wolves.
Chelsea will be desperate to get back to winning ways against Leeds after losing to Liverpool at Wembley at the weekend.
Winning the FA Cup would secure them the Europa League for next season, and European qualification through a league position is far from a certainty.