They paid a lot to Santana.
I'd guess Bour stays only for this season and they flip him in the winter. Doesn't sound like they gave up much to get him, and if they can get his numbers back up by playing him only against RH, they could easily get something better for him in the winter. He's arbitration eligible but has a few years of team control, and honestly he's pretty good. If I were trying to actually win games and not just worry about contracts, I'd play him over Santana except against lefties.