On the eve of his first opening day, on the heels of a breakthrough spring training, Phillies outfielder Mickey Moniak received dispiriting news that will keep him from starting the season on time.

Moniak has a hairline fracture of his right hand, two sources said Thursday, confirming a 93.7-ESPN report, and will miss approximately four to six weeks. According to another source, the Phillies intend to call up outfield prospect Símon Muzziotti, who was slated to begin the season in double A.

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Matt Vierling, who was slated to split time with Moniak in center field, likely will start Friday’s opener against the Oakland Athletics at Citizens Bank Park.

The Phillies thought Moniak avoided serious injury Wednesday after he got hit by a pitch from Tampa Bay Rays left-hander Ryan Yarbrough in the fourth inning of the spring training finale at Tropicana Field. Moniak left the game and went for X-rays, which came back negative. He was diagnosed with a bone bruise.

Moniak, 23, was included on the 28-man opening-day roster that the Phillies submitted to Major League Baseball by Thursday’s noon deadline. But as the team gathered for an optional workout, the former No. 1 overall pick reported worsening symptoms and went for additional imaging that revealed the fracture.

And so, one of the Phillies’ best spring-training stories took a disappointing turn.

“Firm believer in everything happens for a reason,” Moniak wrote on Instagram. “Appreciate everyone’s well wishes it really means a lot. BRB. Till then, this team’s special. Go Phils.”

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Upon reporting to camp last month, Moniak began working with hitting coach Kevin Long to move closer to the plate and change his stride direction. The results were undeniable. Moniak went 13-for-35 (.371) with four doubles, five homers, and a .914 slugging percentage.

It was a stark turnaround. In November, president of baseball operations Dave Dombrowski all but ruled out Moniak as an opening-day possibility even though the Phillies had vacancies in both center and left field. But when center fielder Odúbel Herrera went down with a strained muscle in his side, Moniak played his way onto the roster with the force of an onrushing bull.

“I always knew I would kind of cross-stride and cut myself off,” Moniak said last week. “I’ve done work in the past to try to fix it. But I mean, [Long] got me right from Day 1. I feel like I’m seeing the ball better. I’ve got a lot more time up there to be able to adjust to pitches and still be on the heater.”

Muzziotti and double-A teammate Jhailyn Ortiz were the only healthy outfield options on the 40-man roster. Muzziotti, 23, ranks among the Phillies’ top prospects but played only 20 minor-league games last season because of problems obtaining his work visa.

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Herrera, who was unable to play a spring training game, is slated to start facing pitchers in live batting practice this weekend in Clearwater, Fla. Until he returns, Vierling figures to get most of the playing time in center field. Before Moniak went on his hot streak, manager Joe Girardi considered turning the righty-hitting Vierling loose against left-handers and righties.

But Moniak changed minds with almost every spring swing.

“He’s felt so good, especially when he started working with Kevin,” Moniak’s father, Matt Moniak, said earlier this week. “He’s like, ‘I don’t care where I start. I just want to play ball.’ He already had that mindset.

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“The odds of him making the team and maybe even starting opening day against a righty, what were the odds of that happening? It’s incredible. I’m so proud of him.”

All of which made Moniak’s opening-day eve diagnosis so crushing.