Segura was set to play six innings at shortstop and hit leadoff on Tuesday as well, according to Tacoma play-by-play broadcaster Mike Curto. He could return to the Mariners' lineup as soon as Wednesday's series finale against Detroit if there are positive results.
"Moving around at shortstop is the biggest issue," Servais said of Segura, who sustained a high ankle sprain that placed him on the 10-day disabled list on June 2. "Lateral movement can sometimes be a little challenging coming off an ankle sprain. But workouts have been pretty good. I don't think he's at 100 percent, but a couple games under his belt won't hurt."
Iwakuma was not sharp in his second rehabilitation start, lasting only two innings and allowing four runs on four hits.
The 36-year-old right-hander was slotted to throw 70 to 75 pitches, but he was pulled after 49 pitches.
"Overall, I felt so-so," Iwakuma told the (Tacoma) News Tribune through interpreter Antony Suzuki. "I just couldn't find my command and fell behind in the count. You don't do that if you can command your pitches. I wasn't doing that … it's just not being able to execute your pitches."
In his first rehab start June 14 in Class-A Modesto, Iwakuma surrendered one hit with no walks over four innings with four strikeouts. It was his first game action since being placed on the 10-day disabled list with inflammation in the right shoulder.
Iwakuma is one of the Mariners' three injured starting pitchers. Felix Hernandez, who has been on the DL since April 26 with right shoulder inflammation, is set to return on Friday against the Astros after a successful rehab start with Tacoma on Saturday.
Drew Smyly, who Seattle acquired this offseason from the Rays, is set to throw a bullpen session this week -- potentially Tuesday, according to Servais -- followed by a simulated game. If both of those go well, rehabilitation starts will be mapped out.
Those reinforcements would be key for any run at a postseason spot, according to Servais.
"It's a matter of getting consistent starting pitching, that is what's going to determine [if we can make a run]," Servais said. "I've said that all along since I've got this job. Consistent starting pitching allows you to put the long streaks together. Hopefully we got that coming.
"I think if you add Smyly, who is the final piece we've been waiting for, it could get really interesting."
Dillon Overton, who was designated for assignment by the Mariners on Sunday, was claimed by the Padres and assigned to San Diego's Triple-A affiliate in El Paso, Texas. The Mariners added Max Povse from Double-A Arkansas in a corresponding move.
Overton recorded a 6.38 ERA in nine appearances with the Mariners this season and a 9.33 ERA in seven outings with Triple-A Tacoma.
Josh Horton is a reporter for MLB.com based in Seattle.