Right. So what if Butler is a step slow because he didn't sleep enough, or because he's hungover, or because he's suffering flu like symptoms? It's not like Foles was going to burn anyone with his accuracy, or with the size and speed of his receivers.
The biggest problem is how defense works. The offense throws a lot of weird looks at a defense trying to get them to miss their assignments, a lot of how defenses avoid missing their assignments is knowing what the other guys are going to do. Butler took 98% of the team’s snaps during everybody on the defense knew what he was going to do, take him out, move one guy out of position to cover him and then bring in another out of position guy to back fill that and everybody loses a bit of confidence (which in the NFL is expressed with speed) in what the other guys are doing and what they should.
An Eagles coach has leaked that they completely changed the game plan when they realized Butler was out, because they knew that side of the field was now covered by lesser talent (including a guy they traded to the Pats for a 4th round pick) playing out of position. No matter how you slice it they’d have been better off with Butler.