Joe Saunders is a back-of-the-rotation pitcher who won a bunch of games on a strong Angels team, and in doing so managed to convince some folks that he was something more. He doesn't strike many batters out, and he's not amazing at limiting his walks, either, but he does induce a ground ball just under a 50% of the time. If his defense and his luck is on, he can succeed.
Despite a four-pitch repertoire that looks deep, Saudners actually has a very wide platoon split for a starter, with an awful career 5.01 FIP against righties but a respectable 3.26 FIP against lefties (from FanGraphs). Saunders lives off his two-seam fastball that sits right on 90 mph with very good run. Against righties, his second pitch is the changeup, which averaged 83 mph in 2014 (according to Brooks Baseball). The movement on the pitch looks okay, but he doesn't get a ton of swing and misses with it (he does get GBs), perhaps due to the lack of velocity separation from his fastball.
Against lefties, though, Saunders changes entirely. He basically scraps his changeup entirely and emphasizes his slider instead, which is a putaway pitch (38% whiff/swing in 2014). And while he uses his curve against both lefties and righties, it's much more difficult for the lefties to handle, both creating more whiffs and more groundballs than it did against the opposite-handed batters.
Tonight is a game for the Rays' right-handed platoon players to shine. Saunders both lacks the weapons to attack them, and pitches predictably when facing them (if I was the Texas pitching coach I'd be asking for more sliders early in the count, but old dogs and all). There are no sure things in baseball, but you construct your lineup to give it a chance to succeed, and tonight the Rays should have that chance.