Pena said he hopes to start taking bating practice and fielding drills sometime next week.
"I'm ready," Pena said. "I'm happy and I'm working hard. It's coming back. They can't believe that I'm coming back this quickly."
Pena strained the oblique while taking batting practice at Space Coast Stadium on March 12. Pena was supposed to take five swings in the fourth round of the practice, but had difficulty completing the session. Pena was slated to miss four weeks or more.
"Things are going better than expected," manager Manny Acta said. "[Pena] is healing quicker than what we thought."
Meanwhile, outfielder Elijah Dukes, who has a right hamstring strain, is not with the team. Acta said he had to take care of personal business and will be back with the team for Saturday's exhibition game against the Orioles. Acta said Dukes should be ready to play that day.
Right-hander Tim Redding hasn't had any problems with his back lately, so he will pitch in a Minor League game on Friday. He threw a bullpen on Wednesday without any problems, and he will pitch six or seven innings on a Minor League game on Friday.
Redding will then fly to Washington later on Friday and rejoin the team the next day.
Redding felt some discomfort in his back as he began warming up during the bottom of the fourth inning of Sunday's Grapefruit League game against the Marlins, but decided to gut it out because he was one of only six pitchers the Nationals brought to the game.
Most of Redding's pitches in that inning were left out over the middle of the plate, and the Marlins took advantage, tagging him for four runs.
The pain in Redding's back grew worse when he threw a pitch that went under the glove of catcher Paul Lo Duca for a passed ball. As Josh Willingham scored from third, Redding didn't even make it to cover home plate. He doubled over in pain and called for assistant athletic trainer Michael McGowan to come assist him. Acta and pitching coach Randy St. Claire also headed to the mound. A few minutes later, Redding left the game and was replaced by left-hander Ray King.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.