I agree, he might have a better future as a RP. However, the Tigers do need reserves in the minors at SP to be called up as the 6th, 7th, 8th SP. So with the option available, he's been able to serve as the 6th SP. That was going to be one of Manning, Mize, and Olson, and Manning drew the short straw.
At some point, one of the 1-5 SPs will succumb to injury. Manning will be the fill in this season. Perhaps he establishes himself as a SP going forward. Maybe they convert him to RP after this season. He could also be traded if the Tigers build up enough depth in the minors and/or make some offseason moves. But it seems doubtful that he'll be in the bullpen this season.