Little known fact: Improv comedy has the highest injury and mortality rate of all the comedic styles. Who can forget all the improv related injuries from a broke neck from playing a puppy with torrets at a party sketch or the loss of a right arm from a comedian in the things you don't hear on a porn set sketch or the tragic death of an upcoming comedian who dared to try to sing about construction workers in a 50s do-wop song.
Are we really worried that Gronk will get injured doing "Who's Line is it Anyway?" Have you seen Colin Mockery? Mockery looks like the kind of guy who can throw out his back trying getting off the couch. If that guy can do decades of improv comedy without injury, I think Gronk can do five minutes of it.