Zach Eflin planned on returning to the mound for the Phillies on Thursday night at Citizens Bank Park.

Instead, he couldn’t get out of the training room.

Eflin’s balky right knee “flared up,” manager Joe Girardi said, three hours before his scheduled first pitch, forcing the Phillies to scratch him from a start against the Arizona Diamondbacks. Left-hander Matt Moore, who likely would have entered the game after Eflin, started instead.

“He went through all the preparation it takes to be a starter, and it flared up today and we’re just not comfortable running him out there,” Girardi said. “He thought it would treat out like it does a lot of times and it just didn’t.”

The Phillies reinstated Eflin from the injured list earlier Thursday and designated right-hander Chase Anderson for assignment. It’s not yet clear whether Eflin will need to be sidelined again.

“We’ll just have to see,” general manager Sam Fuld said. “[Friday] will be a really telling day.”

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Eflin has been out since July 20 with patellar tendinitis in his knee. He made a few simulated starts before throwing 31 pitches over 2⅓ innings in a rehab start Saturday night at double-A Reading. Each time, he “responded generally well,” according to Fuld.

The Phillies determined Eflin didn’t need to make another minor-league start before rejoining the rotation. He came through a bullpen session earlier in the week and didn’t have any issues with his knee until after he reported to the ballpark Thursday.

“The body’s a funny thing, and I know that this particular injury has its tendency to be a little fluky and unpredictable,” Fuld said. “I don’t think anybody knows why this happened.”

Eflin has a 4.17 ERA in 18 starts this season. The Phillies are hoping to add him to a rotation that was bolstered by a trade-deadline deal for Kyle Gibson and by Ranger Suárez’s return to a starting role.

The Phillies signed Anderson to a one-year, $4 million deal in the offseason to add depth to the back end of the starting rotation. He posted a 6.75 ERA in 14 appearances, including nine starts.

But if the Phillies had known earlier about Eflin’s setback, they likely would’ve kept Anderson around. By removing him from the 40-man roster, they exposed him to waivers. If he clears, he could accept a minor-league assignment or request his release.

“We never put ourselves in that position to think it through [that Eflin would have a setback],” Fuld said. “It’s all kind of hindsight at this point, but every move or non-move has its consequences.”

Bring back Bohm?

With Rhys Hoskins returning to the injured list, a precursor to likely season-ending surgery for a lower-abdominal tear, the Phillies have a void at first base. Brad Miller, who entered Thursday night in a 5-for-36 rut, got the first chance to fill it.

Would the Phillies bring back recently demoted third baseman Alec Bohm from triple-A Lehigh Valley to play first?

“We’ve seen Alec play over there — last year more than this year — and showed well,” Fuld said. “But we just haven’t gotten there.”

Fuld said the Phillies didn’t have immediate plans to have Bohm play first base in triple A. Bohm started at third base Thursday night against Scranton/Wilkes-Barre.

Alvarado returns

Lefty reliever Jose Alvarado rejoined the Phillies’ bullpen after a turn on the 10-day injured list with shoulder soreness. Alvarado took the roster spot vacated by Hoskins.

Alvarado, whose last appearance for the Phillies came on Aug. 7, allowed one walk and struck out one batter in a scoreless inning Tuesday for triple-A Lehigh Valley.

Extra bases

Center fielder Adam Haseley, who has been in the minor leagues since returning from a leave of absence early in the season, was reinstated from the COVID-19 restricted list and will continue to play for high-A Jersey Shore. Haseley is batting .235 with three homers and a .698 OPS in 112 plate appearances between three minor-league levels, including .220 with a .598 OPS in 54 plate appearances at triple A. ... Outfielder Jorge Bonifacio, who was designated for assignment Sunday, cleared waivers and was outrighted to triple A. ... Aaron Nola (7-7, 4.33 ERA) is slated to start Friday night against Diamondbacks right-hander Taylor Widener (2-1, 4.34).