No one has ever doubted Hunter Greene’s stuff. He came out Monday throwing 100-plus in every fastball. His slider was nasty.

But Greene is also only 23 years old, and Monday was just his 25th big league start. So he still has a lot to learn. Monday’s 5-4 loss to the Pittsburgh Pirates before a regular-season record crowd of 44,063 was a learning experience.

“Today’s a tough day,” Greene said. “We talked from the beginning of spring training leading to today. At the end of the day, it’s one of 162. It’s a very long season. I would have loved to get the first one today. It’s all we talked. All the hard work is going to lead into the rest of the season.”

The critical at-bat of the game came against fellow phenom Oneil Cruz in the third inning. Greene got head ahead 0-2. He was then flagged for a pitch-timer violation.

“Tyler and I were going back and forth on signs. We didn’t get to it quick enough,” Greene said.

The count went to 3-2 before Cruz crushed a home run to right field. Greene said the violation and a borderline call on a Ball 2 did not rattle him.

“I felt like I was locked in the whole at-bat,” Greene said. “There were just a couple of pitches that got away from me. Overall, I was very present.”

Still, he ended up throwing 37 pitches in the inning.

That would come into play in the fourth. Ji Hwan Bae doubled with one out. Greene then lost the ninth hitter, Austin Hedges, to a walk.

Reds manager David Bell immediately brought in Fernando Cruz to face Oneil Cruz.

“If (Greene) would have gotten Hedges, he would have pitched to Cruz,” Bell said. “But 83 pitches in 3 1/3 innings is a lot.”

Greene said fatigue was not a factor.

“I felt great from the first pitch to the last,” he said. “I think anyone who’s performing at this level wants to go as long as they can, especially a starting pitcher. I felt great. I could have gone on. But DB wanted to bring on Cruz. He has a. lot of trust in him so does the rest of the team.”

In retrospect, Greene probably should have gotten another batter or two.

Cruz walked the first three batters he faced. The third walk forced in a run. Another scored on a wild pitch, and the Reds were down 4-1.

They fought back to make a game of it, but they ended a loss. For this Reds team to exceed expectations, they’re going to need more than 3 1/3 innings from Greene. His eight strikeouts were impressive. The three walks and five hits — not so much.

Greene said without really saying that he thought home plate ump Mark Wegner was squeezing him.

“I thought it wasn’t on my end on a couple (of the walks,” he said..” It is what it is. It’s part of the game. Looking at the positives. I threw some really good pitches. It didn’t go my way.”

Greene’s stuff is such that he’s going to be dominant.

“His stuff is unbelievable,” Spencer Steer said. “He’s our ace. When he’s pitching, We’ve got a really good chance to win.”

Again, Monday was a lot for a young pitcher. It was Greene’s first Opening Day start.

“A lot of adrenaline,” he said. “There were times when I was too jumpy. There were a lot of times with that adrenaline that I came out on top.”