SEATTLE -- After missing the final four months of the 2019 season following a ruptured testicle, Mariners right fielder Mitch Haniger had a setback during his offseason training on Monday that will likely require surgery and sideline him for at least the next 6-8 weeks.

General manager Jerry Dipoto said Haniger could miss the start of the regular season following what he described as “core surgery” for a sports hernia, but the team is still waiting to learn more about his situation.

Haniger was working out at the Sparta Science facility in Menlo Park, Calif., and felt an issue when he started ramping up his hitting for the first time this week in his normal build-up for the regular season.

“This is all coming together in the course of the last 40 hours or so, but he’s attacking it very aggressively because he doesn’t want to miss any more time,” Dipoto said. “Mitch was in the midst of what perhaps was his best offseason since he’s been with the Mariners. Mitch is very diligent in his preparation, and all of his metrics, the test scores and athletic explosiveness, were off the charts.”

Haniger was an All-Star in a breakout 2018 season, but he was sidelined last year after fouling a ball off himself in a game on June 6. He hit .220/.314/.463 with 15 home runs in 63 games but was working hard on coming back strong when he re-injured himself this week.

“It’s disappointing,” manager Scott Servais said. “I feel bad for Mitch. He was working his tail off. His strength was up, and he was very excited to get back on the field and get it going again. But I know Mitch will do everything he can to get back as soon as he can. We’ll know more soon.”

Dipoto said Haniger’s injury opens the door for more playing time for prospects Jake Fraley and Braden Bishop. The only certainty now in the Mariners’ outfield is Mallex Smith in center field, while rookie Kyle Lewis is projected to start in left.

“The one certainty is that Mallex will play. Beyond that, it’s a wide-open field,” Dipoto said. “We want to see Kyle Lewis take the ball and run with it, and we expect that will happen. This probably creates more of an opportunity for Jake Fraley to make the Opening Day club in a regular role, rather than moving around in more of a timeshare.”

Promising young prospects Jarred Kelenic and Julio Rodriguez will also get more at-bats now in Spring Training, though the 20-year-old Kelenic is expected to open the season at Double-A Arkansas and the 19-year-old Rodriguez in Class A Advanced Modesto.

Dipoto didn’t rule out the scenario of either playing so well in spring that they force their way onto the team, but that remains an extreme long shot, even with Haniger’s absence.

“Sure, in a very Ken Griffey Jr. 1989 sort of way,” Dipoto said of that possibility. “I wouldn’t anticipate that, because we are dealing with, in Julio’s case, very limited experience above Low-A ball, and in Jarred’s case, just 92 plate appearances above the [Class A Advanced] Cal League.

“I don’t want to do anything that will harm their progress. We want to do the right thing for their development. If the Mariners have an April that is tougher to watch because we were taking our time to do the right thing for Julio and Jarred, that’s what we’ll do.”

Dipoto said he’s looking at the potential of bringing in another outfielder with some Major League experience, while also keeping in mind that Haniger should be back in time for the bulk of the regular season.