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06/10/12 8:13 PM ET

Harper doesn't start, but plays important role

BOSTON -- Nationals outfielder Bryce Harper was not in Sunday's lineup against the Red Sox because of lower back tightness -- "mostly muscular" -- according to manager Davey Johnson.

The injury is not considered serious and there isn't any talk about Harper getting an MRI. Johnson said Harper most likely will be in the lineup on Monday against the Blue Jays.

Harper entered Sunday's 4-3 game victory over the Red Sox in the ninth inning. The score was tied at 3 when the Nationals rallied against right-hander Alfredo Aceves. With one out, Harper entered as a pinch-hitter for Tyler Moore and walked on five pitches. Harper was looking to hit something in the gap or over the wall.

"I was trying to be patient also. I really didn't get anything to hit," Harper said. "I knew I had to get loose and get going. So I went down in the cage and got going."

"I really didn't want to use him late," Johnson said. "He said he was feeling a lot better. He came running out like in the seventh inning. I said, 'Have you been swinging [the bat in the cage]?' He said, 'Yeah, I'm fine.' He made that point clear."

After Jesus Flores struck out, Roger Bernadina came to the plate. Johnson then decided to put on the hit-and-run play with Bernadina at the plate. With Harper running, Bernadina doubled near the right-field line. Once he saw Boston right-fielder Ryan Sweeney slide to get the ball neat the right-field wall, third-base coach Bo Porter knew that Harper was going to score.

"That's when I made up my mind. I was going to send him," Porter said.

"I knew it was going to be a close play at the plate. Thankfully, it wasn't," Harper said.

Harper, 19, has had back issues for at least four days. Johnson would not have known about the injury if not for bench coach Randy Knorr and head athletic trainer Lee Kuntz telling Johnson. Later on Sunday morning, Harper didn't appear to be injured. He was seen jogging in left field at Fenway. He was also seen hitting in the cage.

"Bryce hasn't said anything to me, but we are just being on the safe side." Johnson said. "Normally, I don't have coaches and trainers saying that about players. ... Anybody that has a little something going on, I have to be on the safe side. I give them a day."

What was Harper doing most of Sunday's game?

"Just blowing bubbles [and chewing] bubble gum. I was trying to [watch] the game as much as I could, watch and see if the game gets any closer," Harper said.

Harper's back problems first surfaced on Thursday against the Mets. In the ninth inning, the Mets' David Wright hit a ball to right field. Harper dove and had the ball go by him for a triple. After throwing the ball to the cutoff man, Harper fell to the ground in pain. After the game, Harper said he was OK.

Worth noting

• Nationals reliever Ryan Mattheus is on a rehab assignment for Class A Potomac. Mattheus is on the 15-day disabled list with a plantar fascia strain in his left foot.

Mattheus pitched the eighth inning for Potomac on Sunday afternoon. He took the loss against Salem, allowing two home runs in one inning and throwing 15 pitches in his first action with Potomac.

• In the eighth inning of Sunday's 4-3 victory, catcher Jesus Flores was hit on the right hand on a foul tip by David Ortiz. Flores thought his finger was broken, but he stayed in the game and is expected to play against the Blue Jays on Monday.

"Thank God nothing is serious," Flores said. "[Monday], I'll be able to play."

Bill Ladson is a reporter for MLB.com and writes an MLBlog, All Nats All the Time. He also could be found on Twitter @WashingNats. This story was not subject to the approval of Major League Baseball or its clubs.