"I'm optimistic it's more toward the four-weeks side of things," said manager Scott Servais. "But you never know. It's kind of surprising. I wasn't expecting it to be that long, but our doctors looked at him and thought it was best to put that time frame on him."
The 36-year-old has been one of the American League's better right-handed starters over the past six years with a 63-39 record and 3.42 ERA in 150 games (136 starts). The former Japanese standout was an American League All-Star in 2013, threw a no-hitter in '15 and was a 16-game winner for Seattle in '16, but he's 0-2 with a 4.35 ERA in six starts this season, and his velocity has been among the slowest in the Majors.
Iwakuma was hit by a line drive in his last outing on May 3 against the Angels and had his next start pushed back several days because of a bruised knee. But while he was recovering from that, the veteran informed Mariners officials he was having increased issues with soreness in his shoulder, which had been bothering him his past few starts.
Iwakuma is one of four Seattle starters on the disabled list, along with Felix Hernandez, James Paxton and Drew Smyly. Paxton is expected back later this month, but Hernandez and now Iwakuma won't likely be ready until sometime in June, and Smyly is likely targeted for July.
"It's challenging. I'm not going to lie," Servais said. "It's not something you can ever plan for, but Toronto is having to go through a similar thing. They've got two or three guys out of their rotation. It does happen. You're hoping you somehow just stay afloat until everybody does get healthy."
Cano sidelined third straight game
Pitching isn't the only health issue for Seattle, as second baseman Robinson Cano was held out again Saturday with a sore quad muscle in his right leg and is uncertain for Sunday's series finale as well. Cano took batting practice on Friday, but he wasn't ready to put much weight on the leg.
"When [Cano] was taking BP, he was about 60 percent just using his hands," Servais said. "Pushing off or landing on that leg, it was still tender, so we'll do the right thing today. I want him back at as close to 100 percent as we can, as opposed to sending him out there at 50-60 percent, and now we have another setback and lose him for longer. It's a delicate balance. Obviously we know what he means to our team and lineup, but also we're thinking bigger picture."
• Servais said there was no new update on Hernandez, who will take a few days off from throwing because his shoulder didn't feel great after playing catch on Wednesday as he tries to work back from the bursitis that has sidelined him since April 25.
• Paxton came out of his first throwing session feeling good and "will start ramping it up" as he rehabs a strained left forearm. Servais said Smyly (strained elbow) is expected to start throwing later this month.
• Reliever Steve Cishek (left hip surgery) is expected to make another rehab appearance for Triple-A Tacoma on Saturday and could rejoin the Mariners as early as Monday if things go well.
• Outfielder Mitch Haniger (right oblique) is hitting off a tee this weekend and beginning to progress in his rehab, with a likely target return of late May.
Greg Johns has covered the Mariners since 1997, and for MLB.com since 2011. Follow him on Twitter @GregJohnsMLB. This story was not subject to the approval of Major League Baseball or its clubs.