Those are all valid concerns, but they are also removing 179 innings of Mike Leake's 4.58 ERA from the starting rotation, which should help to improve the rotation's overall stats and increase the average size of the lead the rotation hands off to the bullpen to preserve.
The Reds have decided to have one of their best pitchers pitch more innings and have their weakest pitcher throw fewer innings.
Upgrading the rotation means the bullpen will have more leads to protect and those leads are going to be bigger and easier to protect. So removing the Missile from the bullpen and putting him into the rotation actually makes the bullpen's job easier in some ways.