Left-handed pitcher Luke French was designated for assignment to create room for Pena, who was not on the 40-man roster after being signed by Seattle in late July.
Smoak suffered a broken nose and hairline fracture on the cheek bone right below his left eye on a hard smashed ball by Boston's Jarrod Saltalamacchia in the second inning of Friday's game.
Smoak was seeing another doctor for a second opinion on Saturday evening and manager Eric Wedge said no timeline would be available on his return until after that. But initial X-rays revealed a non-displaced fracture of the nose and a CT scan also showed the hairline fracture of the maxilla bone in his left cheek.
"I was hopeful it was just the nose, but it's a little bit more than that," Wedge said. "He's got a fracture underneath the eye, too. The good news is it's nothing too serious, but we'll still have to give him some time."
Smoak, 24, is expected to be fully sidelined for three to five days before he can begin playing catch and taking batting practice. Wedge said he wasn't certain if the big first baseman would need to wear a protective mask over the cheek and nose.
"I don't know yet. That will be part of the timeline," Wedge said. "We don't want to do anything risky, obviously. I think we all agree with that. So we'll have to see how it plays out in regard to what the doctor says [Saturday night] as well as what we've already heard."
French, 25, was 8-8 with a 6.12 ERA in 23 starts for Tacoma this season. He previously pitched parts of two seasons with the Tigers and Mariners, going 9-12 with a 4.99 ERA in 31 games.