CLEARWATER, Fla. — Seranthony Dominguez has had a setback in his recovery from last year’s elbow injury, casting doubt on the Phillies reliever’s readiness to start the season on time.

Dominguez will be reevaluated Thursday, according to two sources, after informing the team Monday of discomfort in his right elbow. His most recent appearance in a spring-training game was Sunday, when he pitched a scoreless inning against the Toronto Blue Jays.

The severity of the issue remains unclear, one source stressed. The Phillies did hold Dominguez back from throwing for three days, including Wednesday’s off-day for the team. They hope to have more clarity on his condition by the end of the week.

“It’s alarming because it’s him,” a source said.

Last season, Dominguez didn’t pitch after the first week of June because of a strained ulnar collateral ligament in his elbow. At the time, he feared that he would need “a miracle” to avoid Tommy John surgery, a procedure that typically requires a 12- to 15-month recovery process.

But renowned Dr. James Andrews examined Dominguez and prescribed rest and rehabilitation. The Phillies said Dominguez was getting closer to being ready to pitch by the end of last season. He had a normal offseason and was optimistic entering spring training that his elbow was healthy.

Dominguez has made two appearances in spring-training games. Sunday, he struck out two and posted radar-gun readings of 95 mph at TD Ballpark in Dunedin, Fla.

After that game, manager Joe Girardi expressed confidence that Dominguez would be ready for opening day but noted the possibility that he wouldn’t pitch on back-to-back days early in the season.

Health-permitting, Dominguez, closer Hector Neris, and lefties Jose Alvarez and Adam Morgan are locks for the opening-day bullpen. Tommy Hunter will open the season on the injured list, while Victor Arano has not appeared in a game this spring as he recovers from an elbow injury.

At least one of the pitchers in the fifth-starter competition -- Vince Velasquez, Nick Pivetta, and lefty Ranger Suarez -- could wind up in the bullpen. The other spots will be filled from among a group that includes Deolis Guerra, Edgar Garcia, Reggie McClain, lefty Austin Davis, and veteran nonroster invitees Francisco Liriano, Anthony Swarzak, Drew Storen, and Bud Norris.

Dominguez was a revelation for the Phillies two seasons ago. Called up in early May, he tied for the fourth-highest-average fastball velocity among relievers (98.1 mph), posted a 2.95 ERA, and racked up 11.5 strikeouts per nine innings.

But he faltered before the injury. He lost a few ticks on his heater (97.4-mph average) and gained more than a run on his ERA (4.01). His WHIP soared from 0.931 in 2018 to 1.459, and the swing-and-miss rate on his wipeout slider plummeted from 53.8% to 33.8%.

The Phillies are counting on Dominguez to stay healthy, bounce back, and coanchor the bullpen with Neris.

For now, though, they will hold their breath that this latest issue isn’t severe.

NBC Sports Philadelphia was first to report the news of Dominguez’s setback.