Go ahead and get Peralta, but there's no need for a second starter. Add Peralta and you are right back to where you were before Horton went down. Brown, Assad and Wicks have earned the right to prove themselves as long relievers, at least.
I think it's very possible Brown surprises you and develops into a playoff starter over the course of the year.
Peralta, Boyd, Imanaga, Cabrera, and hopefully Steele or Brown shows enough to push one of them out of a spot.