Soccer: Chelsea could be without 18-goal striker Didier Drogba against Portsmouth after the Ivory Coast international hurt his back in a training ground incident today,

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 side.

It will be a bitter blow for the Blues if Drogba misses out as he was outstanding against Everton - scoring two goals in their 3-3 draw last Saturday.

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

"I don't want to change a lot of players tomorrow and we will decide on Didier tomorrow."

Ancelotti side's, caned for their poor defensive performance against the Toffees 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.

The players were booed by a certain section of the Chelsea crowd as they left the pitch after the final whistle, but Ancelotti insists he is fine with their reaction.

"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.

"I think the reaction of some of the fans was normal. We are top of the league and this means our fans always want to see the best. We want to do the best. When supporters pay for a ticket, he can do anything, with respect, naturally.

"To boo is not a problem. It can be a motivation for us."

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," added the Italian. "I certainly do.

"He is a very professional player and maintains a good control when he does the right things or bad things."

Grant will return to Stamford Bridge for the first time since he was sacked after taking the Blues to the 2008 final of the Champions League.

Chelsea lost on penalties to Manchester United in Moscow but Grant then made way for the ill-fated reign of Luiz Felipe Scolari.

"I don't know him," said Ancelotti. "I saw him take Chelsea to the final of the Champions League and for that I think he did a very good job in that season.

"He has arrived at Portsmouth and done a good job, so I expect a difficult game tomorrow."

Chelsea have not won in the last four games but Ancelotti is not unduly worried.

"It is very important to win," added Ancelotti. "We didn't win in the last four games but the team is in a good moment. I don't see a big problem. We have the mentality and the feeling to find a solution immediately.

"I was happy after the Everton game because we played with intensity but we have to find more concentration and focus at set-pieces."

Meanwhile Chelsea insist they are not "hammering" on Pompey's door for the £750,000 still owed to them from Glen Johnson's £3 million transfer in August 2007.

Reports suggested that Chelsea would reject Portsmouth's appeal for more time to pay, but the Blues remain in dialogue with the club over the matter.

Ancelotti also scoffed at reports linking West Ham's England goalkeeper Robert Green with a January transfer window move across London to Chelsea.

"It is not true," he said.