Gera damaged knee ligaments during the FA Cup tie at QPR earlier this month.
The Hungary international sustained a similar injury last season, which
caused him to miss the second half of the campaign.
Clarke said: "Zoltan has had his operation, it went very well and his rehab
starts this week.
"We would expect him to be up and running in time for pre-season training."
Clarke also expects midfielder Youssouf Mulumbu to return from Africa Cup of Nations duty with DR Congo in time to be included in the squad for Sunday's home clash with Tottenham.
Mulumbu is due to fly back in the next couple of days after a 1-1 draw with Mali on Monday ended Congo's interest in the competition.
Clarke said: "We expect him back on Friday and he will be in contention for
"He's just played the three times for his country so there are no problems