Guyton didn't look that good. But, then again, Guyton also wasn't expecting to play most of the game at MLB. He didn't take all the first-team reps all week. Mayo's athleticism allows him to chase down a lot of plays mid-field; I'm sure Belichick has a different game plan when Mayo's out there, just as in the old days they would have a different game plan when a young frenetic Tedy Bruschi was out there.
They'd already gone into the game with what appears to be the clear game plan of "double team T.O., try to double-team Lee Evans, and make Trent Edwards and the running game beat you". With the offense the Pats have, that seemed like a wise plan; score a lot of points, and make Buffalo play time-consuming small-ball. Unfortunately, the game didn't unfold the way they'd hoped; they had a couple of drives killed by 4th-down failures, and then the batted-ball INT for a TD that changed the game. (Not to mention the B.S. roughing call on Wilfork that caused about a 50-yard difference in field position.)
Once Mayo went down, they had to get even more conservative, I'd imagine. Trying to keep the WRs in check, and not having Mayo in the middle, allowed the Bills to move the ball somewhat. But the stats of the game show that the Bills didn't do as great as we thought they did. And, consider, the D-line underperformed and didn't get to Edwards until late in the game (how the Bills' O-line held them off is a mystery) and yet the DBs still did their jobs for the most part.
In short, I think the D is getting a bit unfairly ripped for this first game. (Except for the D-line, which deserves more blame for not making mincemeat out of that Bills' O-line.) The Jets don't have the WRs that Buffalo has, so the DBs will be freer to contribute more towards the line of scrimmage, so I think that Guyton and the D will do a decent job of defending the middle. The question is the D-line. The Jets have a good O-line; if the D-line can get to Sanchez they can rattle him, but if the D-line plays the way they did against Buffalo then Sanchez will have all day long to sit back there and knit a sweater while looking for open targets.