VIERA, Fla. -- The Nationals placed right-hander Tony Armas Jr. on the 15-day disabled list on Monday because of a right groin strain. He is scheduled to come off the DL on April 12, but he will most likely miss four starts, according to interim general manager Jim Bowden.

Armas, who was unavailable for comment, has a Grade 1 groin pull, which is considered mild. The Nationals are hoping that rest and treatment will get rid of the spasms in the groin.

Armas sustained the injury on Sunday, while pitching against the Mets. In the first inning, with one out, he started to feel some pain while facing second baseman Kaz Matsui, the second batter of the inning. Armas managed to get Matsui to ground out, then induced outfielder Carlos Beltran to fly out to end the inning.

Armas walked off the field under his own power but was taken out of the game and replaced by right-hander John Patterson.

"The injury affects the team, but you can't push a pitcher with a groin injury or you'll lose him for the whole year. We have to get the groin right," said Bowden.

According to manager Frank Robinson, Armas took the news of being placed on the disabled list very hard.

"It was a no-brainer (to put Armas on the disabled list)," Robinson said. "Tony always takes it hard. He knows it's for the best. He's very eager to get out there, which I could understand, especially to go through what he did last year.

"Tony is a gamer. He wants to be out there. He feels he could be out there no matter what. You really have to hold him back and do a little of the thinking for him. If not, he would out there on one leg."

It's the fourth consecutive year in which Armas has been placed on the DL. He started the 2004 season on the shelf because of a mild right deltoid strain. In 2003, he missed most of the season with a torn rotator cuff and labrum, and he missed 23 days in 2002 with a strained lower back.

With Armas out, Patterson will move from the bullpen to the fifth spot in the rotation. He will start against the Devil Rays on Friday. Patterson will start only two games during the first three weeks of the season because of the two off-days on the Nationals' schedule.

On Sunday, Patterson made his first appearance since March 15, after missing time because of a right hip flexor strain. He gave up four runs in three innings, but struck out eight and walked none.

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"He made some pretty good pitches. The ball didn't fall his way yesterday," said pitching coach Randy St. Claire. "What I liked was, when he gave up the hits, he didn't start picking with the strike zone. He stayed aggressive with the strike zone."

Although he played just four games this spring, Patterson said on Sunday that he would be ready to start if Armas had to go on the disabled list.

"I don't how many pitches I threw [on Sunday], but they can easily stretch me out to get where I need to be," said Patterson.

Armas' injury also means that right-hander Zach Day will move up from the fifth spot in the rotation to the second slot. Esteban Loaiza and Tomo Ohka will remain the third and fourth pitchers in the rotation, respectively. Robinson doesn't want to mess up Loaiza and Ohka's routine.

"This will keep (Loaiza and Ohka) on their schedule," Robinson said. "If we start moving the guys around, we would have to cut back on their pitch count. This is the easiest way to do it."

Day said that he felt bad for Armas because the fellow righty had worked hard to stay healthy this spring.

"I know he's out there trying," said Day. "I know he wants to be out there. He's probably frustrated as anybody. You hope that his groin gets better. I don't think it will be anything big."

Asked what pitching every fifth day would mean to him, Day said, "It's a different approach. You know when you are going to throw. It gives you the ability to set a routine.

"Sometimes when you are back and forth, you might throw out of the bullpen for two or three days and then (suddenly) you are starting."

Bowden said that Armas' injury, as well as Vinny Castilla's knee injury, forces the Nationals to delay their roster decisions.

"Some guys are making our decisions harder because they haven't stepped up the last few days," Bowden said. "The rain has a lot to do with it, there's no question, and Spring Training is a long time. But, still, we need the guys to step (up)."

While Bowden declined to say which pitchers needed to take that step, the stats show that Patterson, Jon Rauch, T.J. Tucker, Gary Majewski, Joe Horgan and Joey Eischen need to improve on the mound. All six pitchers have been hit hard this spring.