OAKLAND -- A's manager Bob Melvin remains undecided whether to start left-hander Drew Pomeranz or righty Arnold Leon in the second game of Wednesday's doubleheader against the Seattle Mariners.

Leon, 25, who joined the A's on Tuesday, will be activated Wednesday, as teams playing doubleheaders are permitted to carry a 26th player.

Leon is 2-1 with a 4.34 ERA in six starts for Sacramento this season. Melvin doesn't need to know Leon's stats to know what he's capable of doing.

"We've seen his progression the last two springs, and he looked like he belonged more," Melvin said. "He was more confident throwing to big-league hitters."

Melvin said he told Leon that he had put himself in "position to be a viable option for us this year."

Leon has yet to pitch in a major-league game. He said he is doing his best to keep his emotions in check on the eve of his potential debut.

He has had plenty of time to dream and wonder about this moment. He started his professional career in 2006, when he played for Saltillo in the Mexican League.

"I'll be trying to calm down today, tomorrow and during the game," Leon said. "I'm trying not to think about that and I'm just trying to enjoy every moment here."

Pomeranz has appeared in nine games for the A's this season, all in relief. He has a 1.98 ERA for his 13¿2/3 innings. However, the lefty started 30 games during his three seasons with the Colorado Rockies, including four in 2013.

Melvin said Leon is capable of starting or working out of the bullpen. Leon worked as a closer in spring training last spring, Melvin said.