In a very real sense, Trump like was a boxer getting the crap beat out of him this week. He was in grave danger of going down for the count and never getting back up. Even his corner man (Fox News) was starting to think about throwing in the towel as they suddenly found themselves getting beat by MSNBC and CNN in the ratings. If Comey testified convincingly immediately after this onslaught, then it might have been the beginning of the end.
The hiring of Mueller is effectively like an outside force suddenly causing the fight to be temporarily delayed, at least in public. It doesn’t get Trump off the hook, but when you are getting your ass kicked any suspension is inherently good because something dramatic might happen down the road which alters the dynamics of the battle.
Mueller is very unlikely to run this investigation via leaks and he is also not likely to wrap this up real quick. What is NOT known at this moment is whether Comey will still imminently publicly testify, as had been widely rumored to be in the works. There are reports that he still wants to, but there is also speculation that Mueller would not like that to happen and that Comey would certainly cede to his wishes.