The decision comes just one day after Kila Ka'aihue, who was also in the mix for the starting job, made two errors in the A's 12-6 exhibition loss to the Hanshin Tigers.
Defense, Melvin said, proved a big factor in his decision, especially with groundball pitcher Brandon McCarthy on the mound Wednesday against the Mariners.
It's still possible that Ka'aihue -- who, like Allen, is out of options -- will start in the second game against Seattle on Thursday, though his most recent performance didn't exactly help his cause.
Either way, Allen appears the favorite to start the April 6 opener back in the States, yet Daric Barton is a name Melvin isn't quite ruling out.
The A's skipper said he's been getting good reports on the first baseman, who stayed behind in Arizona to get more innings after missing most of the Cactus League campaign while rehabbing his surgically-repaired shoulder.
The A's could conceivably restart the regular season in Oakland with three first basemen on their roster, since they won't need to include a fifth starter in the 25-man count until April 16 or 17.