The Phillies are making an adjustment to their rotation. Rookie right-hander will make his first big league start on Saturday against the As, Matt Gelb of the Athletic. Fellow rookie is set to work from the bullpen. Mercado, 25, just the starting five in late June. He threw five innings of one-run ball against the Cubs in his first MLB start. The Braves teed off on Mercado on Saturday, though, tagging him for five runs on a trio of homers before he could escape the second inning. While one poor start wont change the organizations view on Mercado, most prospect evaluators suggest his fastball-curveball arsenal is better suited for relief work. A former second-round pick of the Rays, Mercado joined the Phils in a minor trade at the beginning of last offseason. He has started 10 of 14 appearances with Triple-A Lehigh Valley. Mercado has a stellar 1.71 earned run average at the top minor league level. However, he has walked more than 11% of batters faced while averaging fewer than four innings per appearance. Phillips has had a more conventional starting role with Lehigh Valley. He has started all 15 Triple-A appearances and averaged slightly more than six innings per game. Derrick Favors Jersey His 4.89 ERA is a lot more pedestrian than Mercados mark, although he has shown somewhat better control (9.4% walk rate). Mercado throws a bit harder than Phillips does and has demonstrated a lot more bat-mi sing ability in the minor leagues. Phillips, whom the team added to the 40-man roster last week, has a modest 19.8% strikeout rate for the IronPigs. To his credit, Phillips punched out seven hitters acro s four innings of relief behind Mercado against Atlanta in his big league debut. The Phillies have built enough of a cushion that they shouldnt be concerned about some short-term uncertainty in their starting five. Theyre eight games clear of the Braves for the division lead and 4.5 games ahead of the Dodgers for the National Leagues top seed. An excellent rotation has been a major reason. Philadelphias front four of , , and is arguably the best in MLB. The fifth spot is the only question mark. allowed a 5.60 ERA over 10 starts before inflammation in his index finger sent him to the injured list a few weeks ago. managed great results as a sixth starter, but he sustained a lat strain thatll knock him out into August. Barring injury, the Phils look like the rare contender that doesnt need to urgently pursue rotation help over the next three weeks. Wheelers status is worth monitoring after the Phils lifted him at 76 pitches during tonights start. The team announced that the star righty was battling lower back tightne s. After the game, skipper Rob Thomson called it a precautionary measure with the Phillies already holding an eight-run lead ( via Alex Coffey of the Philadelphia Inquirer). Thomson suggested the team would evaluate Wheeler tomorrow but indicated the club expects hell be fine to make his next start. Willy Hernangomez Jersey