Chris Sale is for the most part, an open book. He's pretty expressive and honest in interviews, at least enough for us to know that he really, really wants to start. There's also been 94.1 IP of Sale to observe, and there's a general feel of what he does well. It's a lot.
He's got big time velocity from the left side, a big sweeping slider with insane swing-and-miss rates, and a funky arms-and-legs 3/4 delivery that makes him hard to pick up. His sweeping delivery makes him unhittable to lefties (career 2.61 xFIP)--and also raises questions about his durability--but he's held his own against righties (career 3.10 xFIP) more than enough to earn his larger role.
Unlike Brent Morel 's crazed September power surge, Sale's accumulated enough repetitions as a hard fastball-slider type to assume that it's a comfortable identity for him at the major league level. That's a more curious development than is often acknowledged, as Sale's slider grip is something he's developed fairly recently, and his most ballyhooed off-speed pitch coming out of college was his change-up. There were doubts raised about its consistency, and the veracity of Sale's disguising of it, and in a way they were manifested.
Sale's gone to his change only around 7% of the time over his career according to Texas Leaguers. While it was mostly effective, a distrust in it was indicated by its relative absence. Even when faced against righties, where side-arming lefties typically need change-pieces to counter the platoon advantage--and guys like Buehrle and Danks ramp up their change-up use well over 20%--Sale has still only worked up a change usage rate of around 13% for his career.
As J.J. points out, there's nothing that's really been wrong with that limited pitch selection for Sale. While last season featured some oddly memorable instances of Ozzie hiding Sale from righties, he was more than acceptably effective against the 155 righties he faced in 2011 with his bare-bones approach, as his slider is still pretty nice offering to right-handers despite the long look they would figure to be getting at the ball.
Because there's no glaring disparity in the effectiveness of his approach, it makes a little more sense why a projection system like Baseball Prospectus' PECOTA sees Sale's transition to starting full-time going smooth enough to produce one of the 15 best starter ERA's in baseball (2.79).
Despite that, starting isn't relieving. It's only natural that Sale and the Sox are going to look to add it to his repertoire, if not for to make him more viable against righties, than to give him more variety overall.
Since it is Chris Sale and all, he's already a step ahead of the game. Since the start of June 2011, when his ERA was still hanging in the high 4.00's on the year, Sale started throwing his change-up 20.3% of the time to right-handers, producing swings-and-misses a whopping 21.4%, and outpacing his slider.
That's a pretty healthy start. As with any reliever going to the starting rotation, there are still unresolved concerns about his stuff holding up multiples times through a big league order, the horrible workload of it all, and also hoping that Sale's low plane delivery will keep him enough out of trouble to make up for his general lack of precision.
But compared to the rest of the other issues White Sox pitching prospects have to work out, like solidifying a third pitch (Castro, Santiago), solidifying a pitch other than their slider (Axelrod), missing bats (Stewart), or advancing past Double-A (Molina); those concerns seems fairly manageable. Sale has all the tools he'll need for a big league rotation, and has already gotten the best test-run of them possible. And as J.J. also pointed out, since Don Cooper is involved, an effective Chris Sale cutter is probably inevitable.
Prospects are a source of peril that the White Sox rarely tolerate, but Sale offers as much certainty as any 23 year-old hurler could.