NEW YORK -- Nationals first baseman Nick Johnson has been sent to Washington for an MRI on his right wrist on Thursday. He had X-rays taken on Tuesday and the results were negative.

Johnson is listed as day-to-day, but the veteran was seen wearing a soft cast and could be sidelined a lot longer. Johnson strained a tendon in his right wrist during his last at-bat on Tuesday against the Mets.

"I felt a little pain after the swing," Johnson said. "I ended up walking. I went back on the field and it felt tight. I was trying to loosen it up out there. I wasn't going to hit if my spot came up again. It was pretty painful after the game."

This latest setback marks the ninth consecutive season in which Johnson has sustained an injury. The first injury dates back to 2000, when he was in the Yankees' farm system. The most serious injury occurred in 2006 when he broke his right leg. He missed the entire 2007 season because of the injury.

Asked why he has had so much bad luck with injuries, Johnson said, "I don't know. You have the injury taken care of and get back on the field."

With Johnson out, Aaron Boone will get the bulk of the playing time at first base. Boone has been the Nationals' best reserve this season, hitting .319 with three home runs and seven RBIs entering Wednesday's action.

Boone, who is known as a third and second baseman, has made a nice transition to first base. He credits former Marlins first-base coach Perry Hill for his success with the new position.

"I had a great Spring Training instructor last year in Perry Hill," Boone said. "That really helped me as far as learning the position, and then I got to play a lot last year. [Mike] Jacobs went down for a while, so, before I got hurt, I was playing every day there. So I'm comfortable. I enjoy it. Hopefully, Nick's deal won't be long."

Already on the disabled list for the Nats is catcher Johnny Estrada. As a precaution, the Nationals have scheduled an appointment for Estrada to be examined by Dr. James Andrews on Monday in Birmingham, Ala., to gain a second opinion on his right elbow.

On Tuesday, Dr. Ben Shaffer, the team's physician and orthopedist, determined that Estrada has no structural damage in his right elbow, just inflammation.