Nope. There's just no reason right now. In 1995, yeah, maybe he needs to head to the bullpen. But if we're limiting ourselves to "starting pitchers who have no injury risk" than you're going back to the 92mph guys.
TJS brings guys back to about exactly where they were. Horton has crazy good stuff and he's probably going to have 2-4 years between surgeries. Especially if this is more "brace" than "full repair". Horton should be back by around this time (or so) next year.
If it's clear during the rehab he's just not able to start, doesn't have the stamina? Sure, maybe the BP. But right now, even with another unfun road ahead for Cade, he should be given every chance to be a starter.
Put it this way: even if he blows up in 4 years again, a year of rehab mixed in, that's 2.5-3 years. If he is a 2.5-3 win guy those years, he's already at somewhere between 7.5 and 9 wins. It would take him years and years to ever get to that as a reliever.
The game is changing. And maybe we see more piggyback stuff and maybe then the "6 innings a week" he throws is twice in 3+ inning stints. And maybe we can talk about the bullpen. But in the way it's played today, he should come back as a starter. I will admit that's looking at him as a commodity and less as a human, and there's a human factor here too.