Since when is Isner a bye on grass ?!
Well, Isner is born in 1985 and he has so far in his career beaten exactly one player at Wimbledon. Sure, he has beaten the same man twice, but he has not came across as a force to be reckoned with thus far. In other grass tournaments the highest ranket opponent he has beaten is the then 66th ranked Alex Bogomolov.
It could change, naturally. But then it could change also for any unmerited player in the draw. I see no reason to fear Isner specifically.