You have to be under the cap before June 1st. You also have to sign your rookies. You have to restructure every possible contract, have 0 cap space for the offseason and you still might need extra space from somewhere.
The rookies don't have to be signed until June, and the post-June 1 savings can be used to pay for all the in-season cap costs like IR, practice squad, elevations, etc.
It's definitely a possibility, even if it means paying some of Carr's salary (I'm sure paying $5-10m instead of $30m for a guy who won't start will sound good to Benson).