Pena strained the oblique while taking batting practice on Wednesday at Space Coast Stadium. Pena was supposed to take five swings in the fourth round of the practice, but had difficulty completing the session. He is expected to rest and use the exercise bicycle for now.
Pena appeared sad when talking about starting the season on the DL. At first, the outfielder thought he was going to be out for just two weeks.
"It's not good," Pena said. "I have to work hard to come back. They say I'll be out a month. I want to come back before that. I'm going to be thinking about being on the disabled list the whole day. I called my family and they were sad, too. This was my first opportunity to play every day. Now, I have to wait and see what's going to happen."
With Pena out of action, it means that Elijah Dukes will be the everyday left fielder. Entering Friday's first game against the Indians in a split-squad series, Dukes was 8-for-29 (.267) with a home run and three RBIs. But early in that game, Dukes suffered a right hamstring strain and is now listed as day to day. The injury is not considered serious.
Dukes hurt his hamstring in the third inning. With the bases loaded and Dukes on first base, Dmitri Young hit a ground ball to shortstop Danny Sandoval, who threw the ball to second baseman Jamey Carroll to force Dukes out at second.
After he slid into second base, Dukes stayed on the ground for about a minute before slowly getting up. Manager Manny Acta and head athletic trainer Lee Kuntz came to his aid, but Dukes walked off the field on his own power.
"It felt like a cramp," Dukes said. "But I know that I'll be ready within in day or two. It's just a little soreness right now."
Meanwhile, right-hander Shawn Hill played catch on Friday and didn't have any problems with his right forearm. He threw as far as 120 feet. The plan is for him to throw a bullpen session on Saturday.
"I'm trying not to get ahead of myself, when I started feeling too good, then all of a sudden it flairs up on me," Hill said.
Catcher Paul Lo Duca (left knee) is expected to play behind the plate for the first time on Saturday in a Minor League game. He already has already played at designated hitter and didn't have problems swinging the bat.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.