McClendon said the reasoning to flip Iwakuma and Elias was two-fold.
"I want Kuma against Boston, but we also have to watch Elias any time we can," McClendon said. "An extra day just makes sense. He's going into uncharted territory."
Elias is second among Major League rookies in innings thrown going into Monday's start against the Phillies and the Mariners are just bringing him back after a 10-day respite with Triple-A Tacoma to lighten his workload.
Iwakuma and James Paxton will pitch the final two games of the Phillies series on Tuesday and Wednesday, with Felix Hernandez, Chris Young and Iwakuma now set to face the Red Sox on Friday through Sunday. McClendon lined up his pitching the same way last week against the Blue Jays and swept that crucial three-game series in Seattle.
Hernandez, Iwakuma and Young have combined to go 36-15 with a 2.54 ERA in 71 games so far this season. That's the lowest ERA of any starting trio in the AL. The rest of Seattle's starters are a combined 15-23.
Greg Johns is a reporter for MLB.com. Follow him on Twitter @GregJohnsMLB as well as his Mariners Musings blog. This story was not subject to the approval of Major League Baseball or its clubs.