The 23-year-old has been plagued by injuries this season and sustained a second cruciate ligament problem at the start of April.
Campbell had just returned from a similar problem that had kept him out of action for seven months.
The former Manchester United striker will spend much of next season recovering.
He said: “It’s been a few weeks since the operation. I’m back at the start now like last time and I have to work hard to get my knee back to how it was.
“There’s a million different ways to do rehab. The staff here are great and are switching things up to keep me sane.
“Some days we’re on the (Nintendo) Wii and other days we’re in the pool, just keeping it fresh so I don’t get bored or downhearted.
“The biggest part of rehab is always in your head – obviously coming back and having to do it all a second time is going to play on my mind but hopefully I can stay strong.
“At the moment I’ve not set any targets. I’ve still got the leg brace on which will come off next week, so after that I’ll start back on the bike.”
