Pena had started nine games this season, all as the designated hitter during Interleague Play. Pena has been working extensively on his defense over the past week, shagging fly balls and throwing to bases.
He has done enough to convince manager Kirk Gibson that he won't be a liability in the field.
"He's been working out there every day," Gibson said on Sunday. "He's had a lot of good days."
Pena burst on to the big league scene once again this season, belting five home runs in his first 39 at-bats, including a homer in his first game with the club, on June 21. He's been all-or-nothing at the plate, though, hitting .190 but slugging at a .548 clip. He has eight hits, including those five home runs.
Pena has done well as a pinch-hitter, going 3-for-7 in those situations with a pair of homers. In his second game at Chase Field, on June 28, Pena blasted a walk-off, pinch-hit homer to beat the Indians.