Chelsea could be without 18-goal striker Didier Drogba against Portsmouth after the Ivory Coast international hurt his back in a training ground incident earlier on Tuesday.

Chelsea coach Carlo Ancelotti revealed that Drogba had to leave their training session after sustaining the injury which now threatens his participation against Avram Grant's struggling south coast side.

"We had a problem with Drogba," admitted Ancelotti. "He took a knock on the back and I don't know if he will be able to play against Portsmouth."

The Italian added: "I don't want to change a lot of players and we will decide on Didier before the game."

Ancelotti side's, caned for their poor defensive performance in the 3-3 draw against Everton at the weekend, have been working on the type of set-piece situations that have caused them so much trouble this term.

All three Everton goals arrived courtesy of Chelsea's inability to defend properly from free-kicks and throw-ins.

"We have trained and worked on the set-pieces," said Ancelotti. "We have conceded some goals in these situations and we want to improve on that.

"We can do better. We had some problems after the Villa game where we conceded from set-pieces but we did well to improve after that. We have to improve our attention and concentration in these situations."

Goalkeeper Petr Cech has been the target for much of the criticism of Chelsea's poor defending, but Ancelotti continues to back the Czech Republic international.

"Everyone at the club thinks we have one of the best goalkeepers in the world," he added. "I certainly do.