FAYETTEVILLE, Ark. — With his team protecting a two-run lead against the Southeastern Conference’s most powerful offense and his closer unavailable, LSU baseball coach Paul Mainieri approached freshman right-hander Eric Walker to gauge what he had left in the tank.

Walker had thrown 105 pitches in eight brilliant innings to that point. He was ready to throw more.

“I said, ‘Yes, sir, I feel great. Let me go back out there and get it done,’ ” Walker recalled.

And that’s what he did. Walker retired three of the Arkansas four batters he faced in the ninth inning to sew up a shutout and clinch an important series win 2-0 against the SEC West leader.

Mainieri said closer Hunter Newman was unavailable after he threw 29 pitches in Saturday’s dramatic win. He had Caleb Gilbert and Zack Hess ready in the bullpen and ran through possible scenarios in which he would call on them.

He didn’t end up having to.

“Those are the decisions I get paid to make,” Mainieri said. “You make enough right ones, you get to coach a team for a while. If you don’t, they look for somebody else. You figure out who you believe in and you hitch your wagon to those kids and let them go for it.

"Walker’s an amazing kid. He’s a poised young man and a competitor. As long as he wasn’t tired, I thought he was the best guy for us to have out there.”

Walker (4-0) became the first LSU freshman to log a shutout since Alex Lange did so against UNC-Wilmington in a 2015 NCAA tournament regional. Walker has now thrown 16 consecutive scoreless innings.

Until Sunday, LSU did not record a single clean inning in the series, but Walker logged three of them Sunday. He struck out four and only allowed eight base runners.

But he did not do it alone: The defense that LSU (22-11, 7-5) played behind him was top-notch.

Senior shortstop Kramer Robertson had a banner day in the field. He cleanly fielded all seven of his opportunities, and few were routine.

He set the tone in the first inning when he ranged to his arm side to make a backhand stab in the hole, then fired off-balance across the diamond to nip Luke Bonfield.

In the fourth, with runners on the corners, he left his feet to snare a sharp line drive off the bat of Carson Shaddy to end the inning. In the seventh, he corralled another hot shot from Shaddy and whipped it to second base to start a 6-4-3, inning-ending double play.

“That’s my job as a senior shortstop,” Robertson said. “When I have an opportunity to pick us up, I need to do it for us. I don’t look at it as anything spectacular; I just look at it like, that’s my job. I needed to pick my team up there and make the plays. When Eric’s pitching that well, you want to make every play you can for him, especially (against an Arkansas) offense that can explode at any time. They’ve got home run power throughout their lineup.”

Robertson’s double play was the second of three consecutive inning-ending double plays turned by LSU. Walker turned the last one in the eighth when he snared a line drive from Jake Arledge and fired to first base to double off Jax Biggers.

“That fires you up as a pitcher when your defense is making plays like that,” Walker said. “It’s incredible.”

The Tigers didn't muster much offensively, but it was enough.

Catcher Nick Coomes, getting the start after going 2-for-2 off the bench in Saturday’s dramatic win, doubled to the right-field corner to score Josh Smith from first base in the second inning. Greg Deichmann gave LSU some cushion with an RBI single in the eighth. Deichmann finished the series 6-for-13 with a homer and five RBIs.

“We squandered some scoring opportunities that I was really afraid would come back and hurt us,” Mainieri said. “But, hey, when Walker pitches great and we play great defense behind him. … We ended up winning the game. It doesn’t always go by design, but we’ll take it.”

At one point in the series, trailing 8-1 through six innings Saturday, LSU looked destined for its third consecutive series loss. Now it heads back to Baton Rouge just a game out of first place in the SEC.

“We turned it around (Saturday) night, Walker went out and pitched a gem today and now we’re getting on an airplane having won the series,” Mainieri said. “It’s hard to believe.”

Follow Luke Johnson on Twitter, @ByLukeJohnson.