Pena did not play Tuesday in the Cubs' 6-5 win over the Diamondbacks because of a mild sprain of his right thumb, suffered in Monday's game.
"He's day to day," Cubs manager Mike Quade said of Pena. "We could probably use him if I needed to late. We'll be careful. We get a chance to watch Mr. [Tyler] Colvin at first base."
Colvin did just fine, including scooping up shortstop Starlin Castro's throw on pinch-hitter Xavier Nady's hard-hit ball for the second out of the ninth.
"I guess he was ready," Quade said of Colvin.
"I'm ready for wherever [Quade] wants to put me," Colvin said.
Pena injured his thumb in the D-Backs' seventh Monday, when he fielded Gerardo Parra's grounder.
"I made a play, it was a ball in the hole, I think it was in the seventh when [Sean Marshall] came in to pitch," Pena said. "As I threw the ball, I fell to the side and put my glove hand down and I think I jammed it into the ground.
"I told [Quade], 'Do you need me? I'm ready to go,'" Pena said. "We're trying to be cautious. If he needs me to pinch hit or anything, I'm out here working to keep it nice and loose."
Pena said his thumb was more uncomfortable than painful.
"I've been in worse pain before," he said. "I'm available. That was my message to 'Q.'"
Pena said it was a freak injury, and the first time he's jammed the thumb on his glove hand. He knows the athletic trainers will be cautious.
"I always try to push myself too much at times, so maybe it was not rational for me to think I was going to start today," he said. "I feel like tomorrow there's a pretty good chance."
Pena tutored Colvin this spring at first base. Colvin got the start Tuesday, his first in the big leagues there. Before the game, Pena was asked on the possibility of Colvin going 4-for-4?
"I hope he does -- not Wally Pipp me, but I hope he hits four home runs," Pena said.
Colvin delivered one, a two-run blast in the third, and also drew a walk with the bases loaded against lefty Joe Paterson in the seventh, which gave the Cubs a 6-4 lead.
"Off the starter [Barry Enright], I saw how he pitched me the first at-bat, and I tried to go off that and get better timing," Colvin said of the home run at-bat. "A tough lefty coming against me, he's trying to throw strikes. I wanted to see one and he didn't give me one, and I got the walk."
Colvin had only played in the outfield in the big leagues.
"The first time I saw him there, I was like 'No,' but it was my perception, not that he wasn't doing a good job," Quade said. "The more I saw him, the better I felt. Again, lack of experience will make it a heck of a challenge for him. He's willing and has decent hands. It didn't take a second to think about it when Carlos couldn't go."
The extra time will also benefit Pena, who fouled a ball off his right foot on Monday.
"If he walks in here [Wednesday] and says, 'I'm 100 percent,' he plays," Quade said. "I want that to be the case, and I don't want him to come back and have something that's going to linger for weeks or more. We'll take a look at that."
Don't expect Pena to be wearing a guard despite fouling a pitch off his foot. That happened in the sixth on Monday; he hurt his thumb in the seventh.
"[My foot] hurt," Pena said. "After I hurt my thumb, I forgot about [my foot]."