If a doctor (not a head coach or a player), let me repeat, a doctor, clears a player to be able to practice and play on Sunday, then that means the player is healthy enough to play. Plain and simple. I highly doubt that they would risk their career just so the Bills come out with a victory on Sunday.
Every player is 100% in pain in the NFL like Rex said, but injuries are different. This is the NFL and you expect toughness from players and expect them to play through pain. I mean, usually when us "regular folks" have a cast on our arms, we're pretty much limited on the day to day stuff we do. These guys have casts and then go rush the passer still.
At the end of the day, hamstrings are tricky as someone else said but if the doctors said, hey, you're good to go, then my guess is he's good to go.