Gera suffered ligament damage during the FA Cup clash with QPR at Loftus Road earlier this month. The Hungarian international sustained a similar setback 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."
Mulumbu is due to fly back in the next couple of days after his country failed to reach the knockout stages of the competition.
Clarke said: "We expect him back on Friday and he will be in contention for Sunday.
"He's just played the three times for his country so there are no problems there."