"Good to get him back," A's manager Bob Melvin said. "He really got our attention early on. We felt like when we sent him down he was just a little bit tired, and we cut down on his workload and he's been pitching great since."
Melvin said Mengden won't necessarily be restricted to so many innings on Saturday, noting, "I think we're fine with 100 [pitches], depending on how he gets there."
Mengden takes the rotation spot of left-hander Sean Manaea, who played catch on Friday for the first time since departing his Monday start with back soreness.
Mengden will soon be joined in Oakland by a couple of other young prospects, right-handers Jharel Cotton and Raul Alcantara, both of whom could make their big league debuts as soon as next week, according to sources.
Right-hander Jesse Hahn, meanwhile, was activated from the disabled list Friday but optioned to Nashville, and it's uncertain whether he will be given another start for the A's this season, amid a frustrating bout of injuries and inconsistency.
Hahn, who pitched to a 3.35 ERA in 16 starts for the A's in an injury-shortened 2015 season, allowed seven runs on 11 hits in just 3 2/3 innings in a rehab start with the Sounds on Wednesday.
"Jesse had a little bit of a struggle in his last game, so we'll have him pitch a little more down there," Melvin said. "It's been a tough year for him. He's a guy that, for as important as he was to us last year, for him to have to go through a season like this is difficult for him, yet going through some of the issues he's had, hopefully he'll be better for it. We still feel like he has a bright future here."
Jane Lee has covered the A's for MLB.com since 2010. Follow her on Twitter @JaneMLB. This story was not subject to the approval of Major League Baseball or its clubs.