Complex sentence: Although similar to the stress fracture injury Diamondbacks pitcher Brendan McCarthy suffered, it was not deemed as severe, because, as part of the same injury process according to medical opinion, Wacha's injury had not progressed as far as McCarthy's.
Split into simpler sentences: Although similar to the stress fracture injury Diamondbacks pitcher Brendan McCarthy suffered, it was not thought to be as severe. According to medical opinion, Wacha's injury had not progressed as far as McCarthy's.