Sounds like his knee isn't that bad atm, he's playing and is at the WC, but if the scan shows the knee is weak, or had a potential issue that could flare up, that's a big risk, especially how we play, where there's lots of pressing and running.
You cannot know the state of a players knee until you examine and X-ray it, we knew he'd been injured, but he's played a season, but if the scans have shown a potential problem, you can see why we called it off, or at least want to negotiate a deal to protect us, we've had enough crocks recently to be wary!