The right-hander, brought up from Triple-A Nashville earlier in the day to take the rotation spot of an injured Kendall Graveman (shoulder), was taken for three home runs, including back-to-back shots by Carlos Santana and Edwin Encarnacion in a fourth inning he couldn't complete.
"Just made bad pitches," Mengden said. "Every pitch they hit out was up-and-middle."
Mengden's day was stopped short at 72 pitches, after he recorded his only out of the fourth inning, a sacrifice fly to Austin Jackson, who had homered to lead off the third. He was responsible for all of the Indians' runs in the four-game series opener, putting the A's in an early hole.
"He just started finding the middle of the plate," A's manager Bob Melvin said. "Early on, he had a good mix and looked confident. Balls were more on the corners. A few balls caught the middle of the plate, and they had him on the run there a little bit."
The right-hander retired each of his first five batters and seven of his first nine, before coming undone in the fourth inning. He needed 27 pitches to record one out in the frame.
"Can't throw pitches down the middle," Mengden said. "They get hit. Everything else I thought I was throwing pretty well to that point, and then it kind of just snowballed on me."
The quick end to Mengden's debut was compounded in the fourth when he was struck in the left calf by a line drive off the bat of Bradley Zimmer. Mengden, who had allowed two home runs, a double and single, consecutively, to open the inning prior to being struck, lobbied to stay in the game. He exited after Jackson's sacrifice on the next play.
Mengden underwent right foot surgery in February, taking him out of the running for a rotation spot in Spring Training. He was 2-1 with a 2.21 ERA in four starts for Nashville before Oakland recalled his services.
His stay in the A's rotation could be short, with word from the club Monday that rehabbing right-hander Jesse Hahn will be making a rehab start with Class A Advanced Stockton on Wednesday. Hahn was placed on the disabled list Friday with a right triceps strain and has responded well to rest and treatment. He's eligible to return Saturday.
Another A's pitcher on the mend, lefty Sean Doolittle, is scheduled to throw one inning in extended spring training on Tuesday. The next step for Doolittle, who is nursing a shoulder strain, could be a Minor League rehab assignment.
Jane Lee has covered the A's for MLB.com since 2010.