ST. LOUIS -- There was a time Zac Gallen envisioned making his MLB debut in St. Louis as a member of the Cardinals. It isn't turning out exactly that way for the 23-year-old right-hander.

Tonight, Gallen will break into the big leagues with the Marlins, taking the mound in the series finale at Busch Stadium against the team with which he started his professional career.

A third-round pick by the Cardinals in the 2016 MLB Draft from the University of North Carolina, Gallen has been promoted from Triple-A New Orleans because Pablo López, the scheduled Thursday starter, was placed on the 10-day injured list with a right shoulder strain.

"It's kind of funny," Gallen said. "When you get drafted by a team, you think you're going to make it to their stadium in their uniform. But I'm wearing a different uniform. It's awesome. It's everything you ever dreamed up."

Gallen officially will be added to the 40-man roster before the 7:15 ET game, a move the Marlins had been planning to make at some point this season.

Gallen was dealt to the Marlins, along with right-hander Sandy Alcantara, outfielder Magneuris Sierra and lefty Daniel Castano, after the 2017 season as part of the Marcell Ozuna trade.

Ranked by MLB Pipeline as Miami's No. 18 prospect, Gallen has been dominant with New Orleans, posting a 9-1 record and 1.77 ERA. In 91 1/3 innings, he has 112 strikeouts and 17 walks.

"Obviously, he's having a really good year in Triple-A," Marlins manager Don Mattingly said. "Just another young, exciting guy for us. A guy with real nice pitchability. He's got a number of pitches. Can mix and match, and a little bit of power too."

The way Gallen has pitched, it was only a matter of time before he got called up. But in the past few weeks, he saw Elieser Hernandez and Jordan Yamamoto get promoted before him. Those two right-handers had already been on the 40-man roster, making their callups easier in terms of roster flexibility.

"Any time you're pitching well, you kind of want it to happen," Gallen said. "As a competitor, you want to just get in that rhythm and stay in that rhythm. At the same time, you know, you have to wait your turn. Certain factors come into play, other than just performance. I understand that. There are other things that come into play. It's finally here. I'm excited."

The 23-year-old Lopez missed the final month of his 2018 rookie season due to a right shoulder strain.

Lopez is 5-5 with a 4.23 ERA in 14 starts. In 76 2/3 innings, he has 73 strikeouts and a WHIP of 1.12. The right-hander last pitched on Saturday, allowing three runs on seven hits in seven innings in a win over the Pirates.

He returned to Miami on Wednesday and is scheduled to undergo an MRI.

Lopez is the third starter from Miami's Opening Day rotation to be placed on the injured list. He will join Jose Urena (herniated disk) and Caleb Smith (left hip inflammation). Urena is on the 60-day IL, and Smith is on the 10-day IL.

"You want to get ahead of it," Mattingly said. "Especially, the situation we're in, and building this thing, with what we consider is a good, young starting rotation. You don't want to take chances with those guys. You want to make sure you're staying on top of it, and really try to get them back on the field and on the mound as soon as possible, to continue to gain experience. Like Smitty, we wanted to get ahead of it. We feel like his is going good. He will have a rehab start. We feel he's on a rehab track back."

Worth noting
• First baseman Garrett Cooper, riding a 10-game hitting streak, was scratched from Wednesday's lineup due to left triceps soreness.

• The Marlins announced the signing of fifth-round Draft pick, right-hander Evan Fitterer, from Aliso Niguel High School in Aliso Viejo, Calif. He signed for $1.5 million. Miami's eighth-round pick, outfielder Tevin Mitchell, also signed.

• Don Mattingly managed his 556th game with the Marlins on Wednesday night, passing Fredi Gonzalez for the most in Miami history.