Derek Shelton revealed the third piece of his starting rotation Tuesday when he named Mitch Keller to start Sunday against the St. Louis Cardinals at Busch Stadium.

Previously, he said Joe Musgrove and Trevor Williams will pitch the first two games of the season-opening series Friday and Saturday.

Keller threw five innings — four of them scoreless — during an intra-squad game Tuesday night at PNC Park.

“Really excited with how he threw the ball (Tuesday), how efficient he was,” Shelton said.

“The second inning he gave up a couple runs, but the ability to execute pitches (was apparent). Definitely another step of growth for him. Very excited that he will be Day 3 for us in St. Louis.”

Shelton credited pitching coach Oscar Marin and bullpen coach Justin Meccage for helping Keller’s development during spring training 1.0 and 2.0.

“I think (Tuesday) we saw him use all of his pitches,” Shelton said. “We saw the slider later in the outing become really effective. The ability to locate the fastball was sharper.

“This is a young kid who’s going to continue to grow into his stuff. I’m very happy with the pitching group that we have around him to try and help him develop.”

Keller, 24, was the Pirates’ second-round draft choice in 2014 and their No. 1 prospect entering the 2018 season. He started 11 games last season — his first in the majors — and compiled a 1-5 record with a 7.13 ERA, 1.833 WHIP and 65 strikeouts in 48 innings.

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