Good situation for him. Pretty much anyone with a pulse can look effective playing in Jerry Hughes' spot, so if Hughes gets injured and he's next man up he could make himself a lot of money in FA.
Plus, I think he was really held back here because he frequently went too high on the QB (drawing 15 yard penalties), and he gave up gap integrity and opened up running lanes while going after the QB. It's likely that Rex will care less about both of these issues.
I can already imagine a scenario where Hughes gets hurt, and Buchanan steps in on a rotational basis, has a couple of nice flashy sacks while continuing to exhibit the flaws that kept him off the field here. Then a bunch of people go nuts thinking that Belichick screwed up by cutting him because he can do the one thing he does well pretty decently when the opposing OL is paying basically no attention to him. Can we just agree now not to do that?