Just due to the fact that the Sox have had Buerhle should tell you that it's possible to have a long career in the majors without a true plus pitch.
I'm not saying that he'll be elite. I'm saying that for him to be of use to the Sox, all he needs to be is above average to have a long career. Also, I like that Quintana is, for lack of a better word, clutch. Sure, he works his way into bad situations...but most of the time he gets his way out of them.
He's certainly got stones.