Saints wouldn't have risked him if they didn't think he was a hundred percent. But an injury can come through surgery/rehab and then full training - only to blow up on match-day. No amount of stress testing and training can fully simulate the rigours of professional SL.
One only need consider the countless occasions Paul Sculthorpe or Ellery Hanley returned after serious groin injuries only to fall apart before they'd played a handful of games.
They too would have sailed through the same process of post-operative evaluation, full training etc.
I'm not going to rule out some kind of psychological rather than physiological explanation because it's now widely recognised that traumatic injuries such as the one Walsh suffered often leave behind mental scar tissue. At the very least one requires time to feel confident in the integrity of the repair.
I don't want to sound like I'm wishing it on him but I'd prefer to be dealing with some kind of psychological side-effect which we can attempt to treat rather than a serious physiological complication. From what I've seen in collision sports of all descriptions over the years, as the number of "corrective surgeries" rises incrementally, the chance that the player's career is effectively over goes up exponentially.