That's pretty much what we heard from Lombardi and Mayock right before the draft. That his knee issues would most likely affect the second contract and not his immediate situation. And since he is a RB the second contract is less likely proposition anyway.
In short, Michels pick was always about how he can help the team in the immediate future. And since it is absurd to assume that the Pats drafted him in spite of their own doctors warning him any reasonable person can only assume that the professionals whose job it is to diagnose people gave the green light.