It's as simple as the experts identified muscle imbalance as a likely contributing factor in his previous injury. Assuming they understand which way time flows, they believe a muscle imbalance existed before the injuries occured.

Secondarily, between the medical experts and some dude posting on the Internet,

Who had access prior medical records?
Who had access to Watson to obtain information on his training and recovery process?
Who had direct physical access to Watson and was able to perform eight hours of testing (that's a lot)?
Who is more likely to understand the progression and root cause of a muscle imbalance?
Who is more likely to be over confident in themselves?