After a review, replay officials in New York overturned Diaz's call and there were two outs.
Yoervis Medina came on in relief and retired Lorenzo Cain on a flyout to end the inning and preserve the Mariners' 2-1 lead.
"I really didn't think it was that close, I thought he was out live," McClendon said. "That is a big play. We've got problems if he's not out there."
McClendon lost a challenge on an infield single by the Royals' Jarrod Dyson in the ninth inning of Seattle's 7-5 win on Friday night.
Jackson Alexander is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.