Washington Nationals outfielder Michael Morse, seen here at a practice on March 1, has not played in the field for all of spring training. (Jonathan Newton/WASHINGTON POST)

The Washington Nationals could begin the season without left fielder and cleanup hitter Michael Morse after he underwent an aggressive treatment for a strained muscle in his back, the latest blow for a team suddenly besieged by injury.

Morse, who has not played in the field all spring training, apparently received a platelet-rich plasma injection, and his status for opening day is in question, Manager Davey Johnson said Thursday.

“We don’t know exactly when Michael Morse will be back,” Johnson said. “We’re not going to push that to where we exacerbate it. This just gives an opportunity to somebody else. That’s the thing with baseball.”

As Johnson explained Morse’s uncertain status here before a spring training game against the St. Louis Cardinals, back in Viera, Fla., the Nationals sent closer Drew Storen to Washington for tests on his right arm after he felt soreness in his triceps and biceps. The test results came as a relief. Storen should be fine, a person familiar with the results said, and the Nationals just wanted to make sure the soreness did not indicate a deeper problem.

The Nationals are also dealing with Chien-Ming Wang’s left hamstring strain, Adam LaRoche’s bone bruise on his left foot and even reliever Sean Burnett’s back spasms.

Burnett’s injury, which he suffered turning at his locker, is not considered serious; he has had similar flare-ups on three occasions in the past and knows how to manage it. Johnson hopes LaRoche can return to baseball activities by Sunday and expects more clarity after Saturday, when the Nationals conclude a three-game road trip. Johnson had already declared Wang doubtful for opening day, and he expressed similar concerns about Morse.

“It’s probably more iffy with [Morse and Wang] to open the season,” Johnson said.

Morse said — and has continued to say — he is not worried about the injury. But Johnson said the Nationals are neither discounting nor counting on Morse playing opening day.

“To be clear,” Johnson said, “it’s really unclear.”

The Nationals first discovered the injury to Morse’s lat muscle March 6, after he told team trainers he felt discomfort around the bottom of his right shoulder blade. The Nationals stated publicly that Morse would miss only a handful of days.

On March 10, Morse received a cortisone shot. Morse served as the Nationals’ designated hitter March 12 and 13 but never played the field. Morse began playing catch a few days after his injury but could not extend beyond throwing 90 feet. On Tuesday, the Nationals instructed Morse to not participate in any baseball activity for a week.

Johnson was murky in describing the treatment Morse recently underwent but said he believed the slugger had undergone a PRP, or platelet-rich plasma, injection. He called it an “aggressive treatment to help it heal.”

“Then you look at it a week after that treatment,” Johnson said. “He had something to promote healing, and I think it was that — his own blood.”

The platelet-rich plasma therapy, also referred to as blood spinning, is a relatively new treatment for muscle sprains and strains. Doctors remove about 30 milliliters of the patient’s blood and spin the blood in a machine, which separates out the platelet-rich plasma from the blood. The doctors then inject the plasma into the affected muscle to, in theory, promote tissue recovery.

Many high-profile athletes, including Tiger Woods, have used the procedure.Several athletes have raved about the results, but the merits of the treatment are still being debated.

Johnson has been encouraged by Morse’s assertion that he feels fine but also discussed contingency plans for Morse’s absence in left field. He expects Mark DeRosa will begin playing more in left field this spring, and infielder Steve Lombardozzi appeared in the outfield Wednesday for the first time in his career.

Over the weekend, the Nationals signed veteran outfielder Xavier Nady to a minor league contract, and he will play in a minor league game Thursday, his first action with the Nationals. If Morse begins the season sidelined, it will open another spot for Brett Carroll, Jason Michaels or Chad Tracy, the three players competing for the final bench spot. Morse’s absence could also give Rick Ankiel or Roger Bernadina more time in the starting lineup.

In the lineup, the Nationals will need to replace a hitter who last year slugged 31 home runs and batted .303. Johnson said he could move Jayson Werth into the cleanup spot, behind Ryan Zimmerman. “I really like Werth’s approach this year,” Johnson said. “I like where he’s at. I think he’s priming for a big year.” Johnson could also use LaRoche, if healthy, hitting fourth in order to separate right-handers Zimmerman and Werth with a left-handed batter.

With two weeks remaining before opening day, Morse not only has to heal but prepare for the season. Though he has not thrown enough to build strength in his arm, Morse may not have to play in any games in Florida before appearing in the majors. But the Nationals also need proof he has healed.

“First of all, he’s got to get the green light,” Johnson said. “When he gets back, then he’ll start throwing. They want him to start playing catch first. He’s going [rotating his arm in the dugout] and saying, ‘Shoot, I feel great.’ I said, ‘Don’t be doing that. You’re in shutdown.’ When he gets the clearance, we might have to do some more tests.

“We’re just going to be very cautious on him. None of these guys, when they do play, [we have to make sure] that it’s going to exacerbate the injury.”