Alcantara -- ranked by MLBPipeline.com as the A's No. 24 prospect -- allowed eight runs in two innings against the Rangers on Friday, while Hahn managed to hold the Rangers to two runs in the six innings that followed.
"Give Raul a little bit of a break," A's manager Bob Melvin said. "That was a tough one for him. Now, one start and out of the rotation, that's kind of difficult, but with us, these things often flip, when you're trying to play a hot hand."
More changes will come when Sonny Gray, recovering from a lat strain, returns. The A's expect him back by the end of April, setting up an intriguing decision; Alcantara is out of Minor League options.
Melvin said Alcantara, who beat out Hahn for the No. 5 spot in the Opening Day rotation, "just wants to contribute," noting room for improvement with his secondary pitches. Alcantara is expected to be available out of the bullpen as soon as Monday's series opener with the Royals.
"Every now and then, he'll throw a really good splitter, or he'll throw a really good changeup or breaking ball, but it's the consistency with it he's maybe lacking right now, and that can come pretty quickly for him," Melvin said. "We've always thought he was a heck of a prospect, and we still think that, but we're just trying to go with what we think is the best mix at the time, and that's Jesse starting right now."
Hahn was the A's No. 3 starter for much of 2015 before a right-elbow injury ended his season, and he's since struggled to stay in the big leagues. He hopes to replicate Friday's performance, which featured an aggressive approach and a fastball that nearly touched 98 mph.
"That outing was good for me," Hahn said. "It did give me a little bit more confidence. I have to find that within me and take that same approach out there when I'm starting. Really, that's just going to be up to me.
"I always wanted to be in the rotation. I'm definitely more comfortable starting a game, so I'm pretty pumped about it."
Jane Lee has covered the A's for MLB.com since 2010.