Chelsea's Argentine striker Hernan Crespo could be sidelined for up to six weeks with a calf injury, according to the Premier League club's manager Claudio Ranieri.

"Crespo is having treatment and he could be out for a month or six weeks. (Damian) Duff and (Emmanuel) Petit will be back soon but Veron will be out a little longer. He may need an operation," said Ranieri.

Crespo pulled up 10 minutes into the game against Liverpool at Stamford Bridge a week ago, while Juan Sebastian Veron has not played since November due to a back injury and Petit has been out of action since September because of a knee problem.

Chelsea, who face Watford in an FA Cup third round replay on Wednesday, are also without midfielder Geremi, who has left to join the Cameroon squad for the African Nations Cup.

However, Damien Duff could be on the bench after recovering from a dislocated shoulder suffered in December against Fulham.

"Duff I am waiting (on). If the shoulder specialist tells me it is okay, he can be on the bench against Watford," Ranieri told Chelsea's official website.

"Of course, his fitness is not 100 per cent but it is important for him to smell the dressing room, the bench and maybe also the field, why not?"