The resurgence of Lou Trivino could not have come at a more pivotal time for the A’s.

Trevor Rosenthal was signed by the A’s in February with high expectations to replace All-Star closer Liam Hendriks and continue the ninth-inning dominance that became so routine over the past two years. Now that Rosenthal is sidelined for potentially 3-4 months after undergoing surgery for thoracic outlet syndrome last week, the A’s are searching for a reliable option to handle the high-pressure situations that come with closing duties.

It was basically a foregone conclusion that Trivino would elevate himself to the role of Oakland’s closer after he enjoyed a breakout rookie campaign in 2018. He lit up the radar gun with a fastball that flirted with triple digits and possessed a cutter that was widely considered one the nastiest pitches in the game. Then he went through what a lot of talented young pitchers go through — the dreaded sophomore slump.

“He had a phenomenal year and then a lot of times the next year doesn’t go as well for rookies that had a great year,” A’s manager Bob Melvin said before Monday’s game against the D-backs. “The stuff is the same. A lot of times, it’s between the ears and the confidence.”

In 2019, Trivino became enamored with his fastball and cutter, throwing those two pitches 67.4 percent of the time. The end result was a career-worst 5.25 ERA in 61 appearances.

Trivino was doing himself a disservice. Drafted by the A’s as a starter in 2013, the right-hander entered the Major Leagues with a full assortment of pitches — fastball, cutter, sinker, curveball and changeup. Yet he shied away from the offspeed, focusing instead on trying to overpower hitters.

“I have a starter’s repertoire and it would behoove me to not use that,” Trivino said. “It’s a little easier to get through lineups using all five pitches that I have.”

Shades of a return to form for Trivino were evident in 2020, particularly over the month of August, when he posted a 1.35 ERA over nine appearances with 16 strikeouts and just four walks in 13 1/3 innings. After a dominant Cactus League and impressive start to the regular season this year, that renaissance now appears to be in full bloom.

“I feel like the Lou Trivino of 2018,” he said. “I feel like I don’t have to think about things. I’m just attacking right now. Throwing multiple pitches for me has been great. Getting hitters to throttle back and forth between higher velocity and lower velocity. Just mixing it up. I feel consistent right now, and I’m going to ride that wave.”

Through five relief appearances this season entering Monday, the 29-year-old has held opposing hitters to a .048 average (1-for-21) with eight strikeouts and just three walks issued in 6 2/3 innings. The one hit has been responsible for his 1.35 ERA — a three-run homer surrendered to Houston slugger Yordan Alvarez. Other than that one mistake pitch, Trivino has looked unhittable, something his teammates always believed he could be.

“I play catch with him every day. His [stuff] is really good. It hurts my palm a lot,” said A’s left-handed reliever Jake Diekman. “He has a lot of pitches that are very elite. I think it’s harder when you’re younger to realize how good you actually are. I feel like he’s just maturing, where he’s slowly but surely starting to believe he’s actually as good as everyone else thinks he is.

“In ‘18, he was crazy. That’s how he normally is. I’m glad he’s starting to believe the type of pitcher that he is that everyone else is seeing.”

Trivino’s pitch selection is looking a lot more evenly distributed early in the season. His sinker and cutter are both being used 25.8 percent of the time, followed by the fastball (23.7 percent), curve (15.5 percent) and changeup (9.3 percent).

Entering Monday, the A’s had not had a save opportunity through their first 10 games. Combining that diverse arsenal along with his newfound confidence, Trivino may very well be the A’s strongest candidate to close right now.

“He’s feeling pretty confident, as he should,” Melvin said. “We’ve seen a good spring and regular season to this point. With Rosenthal down, we need him.”