Pena strained his left oblique while taking batting practice on Wednesday at Space Coast Stadium. Pena was supposed to take five swings in the fourth round of hitting practice, but had difficulty completing the session.
"When I hit the last [ball], I had to put the bat down and cry. It was [hurting]," Pena said.
The Nationals believe that it will take Pena two weeks to recover from the injury and have not ruled out Pena missing the start of the 2008 season. If that is the case, it would mean that Elijah Dukes would be the Opening Day left fielder.
Meanwhile, Young (strained left side) played in his first Major League exhibition game on Thursday and went 2-for-3 with a walk against the Marlins. Young said that playing in the accelerated camp the previous 10 days had given him the motivation to do well. It was in the camp that Young got into shape for the 2007 season.
"I got to see some of the guys that I was with last year," Young said. "Saying, 'Hi' to them inspires them and it inspires me. Those guys look up to me and know what I've been through."
Mackowiak (abdomen) is currently in the accelerated camp. He played defense for three innings on Thursday morning and didn't have any problems. There is no timetable as to when he will play in a spring game.
"It was a move in the right direction. It's a matter of getting the strength built up to where I can play nine innings," Mackowiak said.
Lo Duca played in the same Minor League game and went 2-for-3. He didn't have any problems with his left knee and is expected to play his first spring game on Saturday against the Dodgers.
"I feel great . I'm trying to get some at-bats. I'm seeing the ball," he said. "I have no pain. I ran the bases afterward and it's great. I got my workout in."
Hill (right forearm) played catch for the second day in a row and was pain free. He threw 50 pitches from 90 feet. Hill will play catch again on Friday and, if he remains pain free, he will have a bullpen session the next day.
"After that, it's play it by ear," Hill said.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.