Forssell has appeared in nine games for Birmingham this season

Forssell, 25, broke down in training at the weekend and required an operation to repair cartilage damage.

"Mikael was feeling some discomfort in the knee for a while and, after a scan, it was decided it needed to be washed out," said Blues boss Steve Bruce.

"It's a shame both for him and for us that he will not be available for between eight to 12 weeks."

One consolation for the Finnish international forward is that it is not the same knee on which he underwent major surgery to rescue his career two years ago.

Forssell has been dogged by knee problems since scoring 19 goals when on loan to City from Chelsea during the 2003-2004 season, when they finished 10th in the Premiership.

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He signed for a second loan season with Birmingham but had to go under the knife in September 2004 after returning to Chelsea.

Bruce splashed out �3m to secure Forssell on a permanent basis in the summer of 2005, but he could only play a bit-part in their losing fight against relegation.

Forssell had been expected to play a key role in Blues' attempt to regain top flight status, but instead has slipped down the pecking order.