Hannemann will report directly to the Mariners, while Frankoff will not immediately.
Hannemann, 26, was rated the 26th-ranked prospect in the Cubs' organization by MLBPipeline.com before the 2017 season. He's struggled this season at the plate, slashing .240/.312/.386 between Double-A Tennessee and Triple-A Iowa. The Kahuka, Hawaii native was a third-round pick in the 2013 MLB Draft out of BYU.
"He's a center fielder, pretty athletic guy, can run, defend and do those other things," Mariners manager Scott Servais said.
Hanneman's acquisition provides necessary outfield depth with Jarrod Dyson dealing with a day-to-day nagging right groin injury that initially landed in on the 10-day disabled list from Aug. 19-31. Servais said on Monday that it's possible he's dealing with a sports hernia or something similar.
"It's something he's going to have to manage the rest of the season," Servais said of Dyson. "You can manage it, and play through it, but you're probably not 100 percent. It's kind of each individual's pain tolerance and threshold and where it's at. We'll see.
"He's a speed guy, a quick-twitch guy, there's no doubt it's a big deal there. But Dyson running at 85 percent is pretty fast. Faster than most our guys at 100 percent."
Phelps done for the season
Phelps, who the Mariners acquired from the Marlins in exchange for four prospects in July, is being shut down for the season after placed on the disabled list with right elbow impingement for the second time since being brought to Seattle. He pitched only 10 games for the Mariners.
Servais said he might require a minor surgery to clean up his right elbow.
"I think that's the smart thing to do now before it turns into something bigger," Servais said. "He's frustrated by it. You know, before it happened he was throwing the ball great. I thought he was going to be a key piece for us down the stretch. Obviously, it's not going to happen and we're going to have to lean on some other guys."
Frankoff, 29, owned a 4.40 ERA in 23 appearances (21 starts) in Triple-A Iowa this season. He pitched one inning for the Cubs in his Major League debut this season, allowing two runs on four hits in two innings.
Curtis, 25, pitched 4 2/3 scoreless innings over three appearances with the Mariners this season.
Josh Horton is a reporter for MLB.com based in Seattle. This story was not subject to the approval of Major League Baseball or its clubs.