The Finn will have more surgery next week to repair the problem.

Niemi told the club's official website: "I'm extremely disappointed that having returned to training, I now have to undergo further surgery.

"However, I'm confident that I can recover from this setback sooner rather than later.

"I was looking forward to joining the new players who have been added to the squad but I will now focus on my rehabilitation after surgery and look forward to returning as soon as possible."