WASHINGTON -- Nationals first baseman Nick Johnson was evaluated by Dr. Richard Berger, a hand and wrist specialist at the Mayo Clinic in Rochester, Minn., on Monday.

Berger said that Johnson needs arthroscopic surgery to evaluate the ligaments on the ulnar side of his right wrist. Johnson will have the surgery on Tuesday at the Mayo Clinic.

The Nationals haven't ruled out Johnson being out for the year. If that's the case, it will mark the fourth time in five years that Johnson didn't finish the season because of an injury.

"They are going to go in -- look and see what it is," general manager Jim Bowden said. "It has been a tough year for all of us with the injuries. Obviously, it's a setback. It's a disappointment."

On May 15, the Nationals placed Johnson on the 15-day disabled list with a torn tendon sheath in his right wrist. Two weeks later, Johnson had a cast taken off, but he still felt soreness and had the cast put back on. The cast was taken off again last week and Johnson still felt pain.

Meanwhile, right fielder Austin Kearns (right elbow) took batting practice and went through outfield drills on Monday at Nationals Park. He didn't appear to have any problems. He was seen throwing accurately to second and third.

If things go according to plan, Kearns will start a rehab assignment in Class A Hagerstown on Thursday and then go to Triple-A Columbus on Saturday. When he comes back, Kearns will return to right field, while Elijah Dukes most likely will be the everyday left fielder.

"Once I loosen up and everything, it feels good. Hopefully, it feels good tomorrow," Kearns said. "It's not fun watching. I want to be out there."

Reliever Chad Cordero (right shoulder adductor tear) has started to pitch on the mound. Bowden saw Cordero last week in Viera, Fla., and Bowden believes Cordero is in the best shape of his career and looked sound mechanically. Cordero is expected to be activated from the disabled list after the All-Star break.

Left-hander John Lannan had both an MRI and a CT scan on his right wrist on Monday morning. Neither test revealed a significant injury. He is being treated for a soft-tissue contusion. Lannan will not miss his next start.

On Sunday, Washington received a scare in the fifth inning when Jarrod Saltalamacchia's line drive hit Lannan in the right wrist. However, Lannan remained in the game for another inning.