PHILADELPHIA -- The Pirates came into this season with questions about how much starting pitching depth they have. Now it might be time for that depth to be tested.

The team placed left-hander Marco Gonzales on the 15-day injured list on Sunday with a left forearm muscle strain. While the severity of the injury is not yet known, Gonzales missed most of the 2023 season with a forearm injury. He had surgery in August to decompress the anterior interosseous nerve in that forearm.

“He came out after the game and expressed to [Rafael Freitas, head Major League athletic trainer] that he was having some soreness and under the abundance of caution, especially with Marco’s injury history, we wanted to get him back to Pittsburgh and get him reviewed,” said manager Derek Shelton before Sunday’s game against the Phillies at Citizens Bank Park.

Gonzales was one of the team’s more notable offseason additions, when Pittsburgh acquired him at the Winter Meetings from the Braves, who picked him up as part of a trade with the Mariners earlier that week. While his injury caused him to have a down season in 2023, he was a reliable arm out of the Mariners’ rotation for years. From 2018-22, his first five seasons in Seattle, he went 56-45 with a 3.94 ERA over 765 2/3 innings.

His Pirates tenure had also started off very good, as he allowed just five runs over 17 innings in his first three starts with his new club. That included a six-inning, two-run performance Saturday against the Phillies, when he had all of his pitches working.

“I think this is, as far as execution-wise and movement on my pitches, this is some of my best execution,” Gonzales said after his start Saturday. “I'm hoping to stay within that."

That’s going to be on hold for at least the next 15 days. While the team and the player learn more about the injury, the Pirates are going to need to pivot away from their original starting five pitchers.

“I think we need to get through this off-day,” Shelton said. “I think we're in good shape through the off-day [Thursday] with where we're at. I think we'll get through the off-day and kinda review from there how we're going to fill that spot, if it's going to be traditional or non-traditional.”

Because of their off-day Thursday, the Pirates don’t need a fifth starter in their rotation until April 23. When that spot comes, they should have several options. Quinn Priester is doing a good job missing bats in the Minors, striking out 20 over 13 2/3 innings for Triple-A Indianapolis thus far. Veteran lefty Eric Lauer signed a Minor League deal late into Spring Training, and while he would need to be added to the 40-man roster, he’s in position to throw four or five innings. If the Pirates want to roll with a nine-man bullpen for a bit and have a bullpen game instead of a fifth starter, Luis Ortiz, Josh Fleming and Roansy Contreras are stretched out enough to go multiple frames.

However, No. 1 prospect Paul Skenes is not currently in the mix to take that spot. The Pirates have been very deliberate with his workload and build-up for the season, and he’s just not stretched out enough yet. He threw 55 pitches over 3 1/3 innings his last outing Friday, both of which are highs in his young professional career.

“We don’t want to go from zero to 100 right away,” said general manager Ben Cherington on his 93.7 The Fan radio show Sunday afternoon. “... We want to take it a step at a time.”

• Skenes notches career-high 8 K's, continues scoreless run at Triple-A

For now, the Pirates have recalled right-handed reliever Ryder Ryan to take Gonzales’ spot on the 15-man roster, so they will have a larger bullpen for at least the short term.

The best-case scenario for the Pirates and Gonzales is they learn more about the injury and it turns out to be nothing serious. Time will tell if that’s the case.

"He's been really good,” Shelton said. “I mean, even yesterday, late in the game, he was really effective. I think I said postgame that I thought his last two innings were his best two innings. It's one of those things where let's get it looked at, let's get it reviewed and not jump ahead toward anything. We love to live in a speculative world today, so let's just get through it, see where we're at and go from there."