Because all injuries are the same severity right? By all accounts this is nothing more than a pain management issue and there is virtually no chance to injure it further outside of being hit on the same area in the same way again, which is unlikely as it's been 4 years between occurrences.
Firstly, ANY sprain is a form of tear. Can be anything from a microtear(what Allen likely has) to a full year.
Secondly, the UCL is not utilized much in throwing a football. The Ulnar Nerve is likely a bigger concern as that would effect his ability to grip the ball(why he "dirted" the next throw 3 yards in front of Diggs).