"We wanted to take a harder look at Arnold," Melvin said. "He's been up here a couple times, hasn't really gotten an opportunity to be in a prominent role. It's just been one-inning stints for the most part."
Leon has spent time as both a reliever and starter in the Minors, but he's come out of the bullpen in all seven of his big league appearances.
This year he has no decisions and a 3.72 ERA out of the 'pen. He was 1-4 with a 3.12 ERA in 17 games (six starts) with Nashville.
Melvin said he expects Leon to be used "just in front of the length role."
"Organizationally, I think they look at him more as a reliever at this point," Melvin said. "He definitely has the ability and the repertoire to do both, but at this point in time we want to look at him harder in the bullpen."
To make room for Leon, the A's optioned right-hander Dan Otero (2-4, 5.79 ERA) to Nashville.
• Left-hander Sean Doolittle (strained left shoulder) will throw a simulated game Tuesday, which could be his last step before a rehab appearance.
The setting will be more realistic than the pain-free bullpens Doolittle threw this past week.
"Try and get the adrenaline flowing a little bit more throwing to hitters," Melvin said.
• Outfielder Josh Reddick was out of the lineup Monday after leaving Sunday's game against the Indians with right lower-back tightness. Melvin said Reddick's availability would depend on how the right fielder looked before the game.
Trevor Hass is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.