Though the full extent of his left thumb injury remains unclear, the Nationals placed left fielder Bryce Harper on the 15-day disabled list with a thumb sprain after he visited a hand specialist and received an MRI on Saturday.

Harper will see another specialist — Dr. Thomas Graham of the Cleveland Clinic — for a second opinion on either Monday or Tuesday, which should determine the complete nature of Harper’s injury and how long it will keep him out. The Nationals’ worst fear is ligament damage of the kind that shelved Los Angeles Angels slugger Josh Hamilton for six to eight weeks.

“Doesn’t look good,” one person familiar with the situation said.

Harper stayed upbeat, if disappointed. He remained hopeful that the swelling in his thumb could decrease in time for him to miss minimal time. Harper pointed out that the Nationals’ schedule calls for several ensuing off days. If Harper returns May 11, the first day he would eligible, he would miss only 10 more games, Sunday included.

“Hopefully, the 15 days go back and I’ll get back out there,” Harper said. “We’ll see how I feel in 15. … I’m not really worried too much. I’m just trying to get my treatment and see where I’m at.”

Still, the Nationals have reason for concern. Harper, 21, sprained his thumb in a similar manner in high school. Manager Matt Williams said there appears to be laxity along with swelling in Harper’s thumb, potentially a product of his high school injury. The second opinion in Cleveland will help determine which damage in Harper’s thumb he has played with for years, and which damage is new.

“We fear that he’s got a really bad problem with the thumb,” Williams said. He added later, “the prognosis, we just don’t know yet.”

After Harper visited a hand specialist Saturday at Nationals Park, the doctor ordered an MRI and the Nationals feared Harper may have suffered ligament damage. Television cameras spotted Harper wearing a small cast on his hand during the Nationals’ 4-0 victory over the Padres.

Harper injured his thumb Friday night when he slid headfirst into third base, after his three-run triple gave the Nationals a 5-0 lead. Many have questioned Harper’s dive, but Williams dismissed the idea and blamed only misfortune.

“I think it’s bad luck,” Williams said. “It happens all the time. Guys slide headfirst all the time. It just happens that he got his thumb in an awkward position. You can’t question that. You can’t question the way the kid plays the game. He plays the game hard.”

After a slow first week to start his season offensively, and despite a controversial benching for what Williams deemed “lack of hustle,” Harper has hit .345/.415/.534 in his last 16 games.

For however long the Nationals miss their 21-year-old dynamo, they will replace him with a combination of Kevin Frandsen, Nate McLouth and Tyler Moore, Williams said. As Harper missed Saturday’s game, Williams played Frandsen in left field against a right-handed starter, even though McLouth, the left-handed half of that platoon, was fully healthy. Sunday, McLouth is in the lineup against right-hander Ian Kennedy.

The Nationals signed McLouth in December expressly for this contingency. Both Harper and Jayson Werth missed time on the disabled list last season, the Nationals wanted an over-qualified fourth outfielder in case of another injury. Though he has started 0 for 31, peripheral statistics, such as his .120 batting average on balls in play, suggest he will rebound.

“The way [Harper] plays, things like that can happen,” McLouth said. “And I mean that in a good way. It’s unfortunate. He plays in such a way that things like that can happen. It’s kind of a freak thing. His finger kind of caught on the base. Hopefully, I’ll be able to hold down the fort until he gets back.”

Harper’s injury adds to the Nationals’ confluence of April health issues. Right-handed starter Doug Fister, third baseman Ryan Zimmerman, catcher Wilson Ramos and reserve outfielder Scott Hairston have all already landed on the disabled list with injuries that will cost them more than a month.

“It’s frustrating,” Harper said. “I want to be out there and help the team every single day. We got a lot of young talent. We got a lot of great guys. Souza is a damn good player. We’ve got a lot of guys coming back also in the next couple weeks.”

Harper will land on the disabled list for the second straight season. Last year, Harper missed 31 games with bursitis in his left knee, an injury he played through for much of the season and had repaired with offseason surgery.

“It sucks, definitely,” Harper said. “It’s part of baseball. You slide into the bag, and sometimes you catch your fingers. It’s just part of it. We’ll see where I’m at in 15. I really hope I can get back at fast as I can.”

Harper’s roster spot be taken by Steven Souza Jr., who flew to Washington on Sunday morning. Souza joined the Nationals earlier this month for his first stint in the major leagues after center fielder Denard Span went on the seven-day disabled list with a concussion. After his first call-up, Souza missed his flight to Atlanta. This time has been far more smooth.

“Those kind of first-day jitters where you don’t know anything — you don’t whether to put butter on the toast or what — are kind of gone,” Souza said. “So it’s more of, how are we going to win this game? How can I help this team? How can I be a part of this? I’m kind of focusing on that.”