ACL's anymore are not that bad in the NFL. The only two players I've seen struggle with it were RGIII (tried to come back too soon plus had a switch in how they were trying to play offense which didn't fit his skillset) and Houston of the Chiefs (his was weird because it didn't tear but just stretched out and so doctors didn't know whether to operate or see if it would heal itself and well it didn't leading to a late surgery).
He said recently he is already running on it so with 4-months until the season kicks off I have very few doubts that he will be ready week 1. Now maybe he misses part of training camp so they needed another arm for practice is all I can see with this type of move.
He'll definitely be ready to go for the regular season opener. All they're doing is providing depth for the bottom of their QB depth chart. Callahan will be cut before the end of the preseason if not by early training camp.