Bryce Harper endured a similar injury to his thumb while in high school. (John McDonnell/The Washington Post)

The Washington Nationals suffered their most devastating blow Monday in an opening month packed with injuries when they learned left fielder Bryce Harper will undergo surgery to repair a torn ligament in his left thumb and could miss two months, according to multiple people familiar with the situation.

Harper visited orthopedic surgeon Thomas Graham at the Cleveland Clinic on Monday for a final opinion on his battered left thumb, which had already landed him on the Nationals’ crowded disabled list. The diagnosis confirmed the Nationals’ worst fear and will leave their already diminished lineup without one of its best players until roughly the start of July.

After Saturday, the Nationals worried Harper had damaged his ligament sliding into third base Friday night with a three-run triple. Harper, Manager Matt Williams said Sunday, endured a similar injury while in high school, which added to the concern.

Despite a slow first week and a widely debated benching for what Williams termed “lack of hustle,” Harper had started to find his offensive stride. In his past 65 plate appearances, Harper had hit .345 with a .435 on-base percentage and a .534 slugging percentage.

Harper, 21, will miss a significant chunk of a season for the second time in his young career. Last year, the left knee bursitis he played through for much of the year forced him to the disabled list for 31 games and ultimately necessitated offseason surgery.

Doug Fister joined the Washington Nationals this winter after a trade with the Detroit Tigers. A left-handed batter and right-handed thrower, Fister will be an asset to this season’s starting pitcher lineup. (Whitney Leaming/The Washington Post)

After Harper ran into the Dodger Stadium fence last year and exacerbated his knee injury, many questioned if his style of play would allow him to last. After he slid headfirst Friday, the same chorus rose, this time to ask whether Harper would have chosen a safer tack if not for Williams’s benching. Sunday, Williams dismissed the injury as random misfortune. After all, how many headfirst slides result in no harm?

“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.”

With Harper out for an extended period, the Nationals will rely on Nate McLouth, the veteran the Nationals signed to a two-year, $10.75 million contract in December. They paid a premium for a backup because both Harper and Jayson Werth had missed significant time in recent seasons, and last year they had no suitable replacement.

“We thought with Nate, who’s been a really good player on a contending team, he can be an everyday guy,” General Manager Mike Rizzo said Monday afternoon, before the Nationals had received the results of Harper’s tests.

Rizzo envisioned a platoon of McLouth and Scott Hairston forming a left-right platoon in the event one of their corner outfielders went down. With Hairston still recovering from an oblique strain, Tyler Moore and Kevin Frandsen could see time in left field, too.

McLouth has started 4 for 34, including his first homer Sunday. McLouth has insisted all month his at-bats have been better than his results, and peripheral numbers suggest McLouth is poised to break out. He has struck out six times and drawn six walks and seen 4.21 pitches per plate appearance. McLouth has a .111 batting average on balls in play, a sign of rotten luck.

“Hits aren’t falling yet,” McLouth said Sunday. “But I’ve felt like I’ve had competitive at-bats. It’s not like I’m going in there rolling over the first pitch or striking out a bunch. . . . I have felt lost the plate before. I haven’t felt like that.”

The Nationals have already endured injuries to one-fifth of their desired opening day roster. At the end of spring training, new right-handed starter Doug Fister strained his lat. Catcher Wilson Ramos broke his hamate bone opening day. Ryan Zimmerman broke his thumb April 12, diving back into a base. Hairston went to the DL in the season’s first week. The Nationals also missed center fielder Denard Span for one week with a concussion.

“It’s not good,” first baseman Adam LaRoche said. “There’s no way around it. Anytime you lose one guy from the lineup, then it’s a little bit of a blow. You lose three kind of middle-of-the-lineup guys, it makes it hard. I can’t remember two years ago, but I know we had some pretty key injuries and guys stepped up. That’s going to have to happen again.”

Rizzo’s response, to himself and his team: Deal with it. He added McLouth, Frandsen and Jose Lobaton in the offseason as insurance for injuries, and he expects the Nationals will win.

“We felt that we addressed that in the offseason,” Rizzo said. “We never panicked. You haven’t heard me talk about it or use it as an excuse. Nobody is going to feel sorry for us. It’s never good when you lose your 3-4-5 hole hitters in Ramos, Zim and Harper. There’s games to be played and there’s games to be won. That’s why we prepare for these things.”

By the Nationals’ next homestand, which starts May 5, Ramos, Fister and Hairston may have all returned. They will not be able to make up for the loss of Harper. But as the Nationals have learned the past two seasons, the games will not stop for them.

“Slowly, we’ll get a team that we thought we were going to have in spring training at the same time,” Rizzo said. “When we do that, I think we’ll start playing more consistently and getting on a roll. I haven’t mentioned it [injuries], because it’s not relevant. Everybody gets them. It’s time to strap it on and win some games. I feel really good about our lineup and pitching staff and our bullpen. With all the things that have happened, I’m satisfied.”

James Wagner contributed to this report