McGowan is out of options so it's more practical to swap him with someone already in the bullpen than to call someone up. The bullpen makes more sense with a hard thrower like McGowan replacing one of the long men anyways.
No one really likes Happ or his gripping about the rotation but when the guy who essentially took his job flat out states that he doesn't have the stamina to be a starter, his gripes start to sound pretty legitimate. If Happ is here then it should mean he's healthy because as many have pointed out, he still had rehab time to work with (which by the way a team should not use for any reason other than rehab unless they like attracting legitimate union grievances). A healthy Happ in all likelihood isn't the 20 ERA gas can we saw in the spring and more like a 4.5 ERA 5th starter. If he's less than that well the team might as well find out what's it's paying for regardless. The team has enough depth at the moment that it doesn't have to feel tied to one option in the 5th starter. Might as well hold try outs until you find someone who can eat some innings whether that's Happ, Redmond, Rogers or whoever.