Iwakuma threw four scoreless innings with three hits, one walk and four strikeouts in 68 pitches against Reno in his second rehab outing since straining the lat muscle behind his right shoulder in his third start of the season with the Mariners on April 20.
"He continues to improve and move forward," McClendon said. "His first two innings were kind of tough, a little erratic, and his mechanics were off, but he seemed to settle down and threw a little better in the fourth inning.
"He's progressing. He's healthy. He touched 91 mph a few times and was at 87-91, so that's encouraging."
If Iwakuma is deemed ready to return after his Tuesday start, he'd be in line to rejoin the rotation about when the Mariners return home to start a seven-game homestand before the All-Star break. That raises an interesting situation, however, as Roenis Elias filled in extremely well for Iwakuma before hitting a rough stretch his last three outings.
Elias is 4-5 with a 4.25 ERA in 12 starts and young lefty Mike Montgomery has been even better (2-2, 2.04) in five starts since replacing the injured James Paxton.
"That's a nice problem to have," McClendon said of the rotation depth. "Right now we're still in the process of figuring out what we're going to do when he comes back. We don't have those answers yet."
• Paxton played catch at Safeco Field on Friday in his first throwing session since going on the 15-day disabled list on May 29 with a strained middle finger on his left hand. Paxton will need to build up his arm after missing a month and seems unlikely to be ready until late July or August at the earliest.
• McClendon said the plan at this point is to keep all his starters in line after Monday's off-day and just give everyone the extra day off, which would mean Montgomery and Elias facing the Padres on Tuesday and Wednesday in San Diego's two-game Interleague series, followed by Taijuan Walker, J.A. Happ, Felix Hernandez and Montgomery in a four-game set in Oakland to end the trip.
That would set up Hernandez to pitch once more before the All-Star break, on July 8 against the Tigers at Safeco Field. The Mariners' ace then would be in line to pitch five days later in the All-Star Game in Cincinnati, where he seems a sure-fire choice for the American League squad with his 10-4 mark and 3.24 ERA. Hernandez started the All-Star Game last year for the first time in his career.
• Major League Baseball made a scoring change from Montgomery's last start, his 7-0 complete-game victory over the Royals on Tuesday, charging Montgomery with one more hit -- and a five-hit shutout -- and taking an error away from first baseman Logan Morrison.