The Nets expected Jarrett Allen to spend much of his rookie season in the G-League, but he has gone from key reserve to entrenched starter.

But if they’re going to dig themselves out of the Grand Canyon-sized hole they’re in, Allen can’t just be good. He has to someday become great, and for that he will have to become bigger, stronger and a better rebounder.

No pressure on a spindly teenager.

“Jarrett Allen, he’s got to be not a good player but be a great player,” said coach Kenny Atkinson, who had no problem picking out Allen’s greatest strengths — and his most glaring weakness. “Maturity. For a 19-year-old kid, it’s amazing how calm he is, how composed and intelligent. And he has a quiet chip on his shoulder.

“He’s got a lot of pride. We challenge him, he’ll answer, which means you can coach him, which is huge. And then he’s got all the other things that Sean [Marks] and his group saw when we drafted him: Athletic, great roller, rim protector. Now we need him to rebound. He’s got to pursue that thing better. To his defense, he’s coming over to block a lot of shots, but he can make strides there.”

Allen is averaging 8.2 points and 5.3 rebounds, including 10.9 points and 6.3 rebounds in 27 games as a starter. But that rebounding figure is just 46th among centers, a weakness Atkinson will prod him to work on this offseason.

“He’s just trying to bring out another side of me, encourage me to be the best player I can be. He’s just going to find a way to do that and I think he’s doing a great job so far,” Allen said of his rebounding. “I think it’s going to be both. Strength is going to make it easier, and if I learn tricks then it’s going to help with the size.”

Allen’s 9.7 rebounds per 36 minutes are tops on the Nets roster, ahead of Rondae Hollis-Jefferson and DeMarre Carroll. And with his modest 234 pounds. spread over a wiry 6-foot-11 frame, he’ll naturally gain more power and girth. But strength isn’t the only key to improving his boardwork.

“I think strength is one part of it, but we can’t use that as an excuse. He has to be more assertive,” Atkinson said.

“Going after balls, rebounding out of his area a little more, hitting first, getting a hit on the player and then going and getting it. We call it “hit-find-fetch,” which we stole from [Michigan State coach] Tom Izzo. He’s got to get all three of those together. Sometimes he goes up with one hand and it kind of squirts away. He’s just got to get better there. Part of it is mentality; go after that thing. Go get it.”

DeMarre Carroll (left hip strain) and Isaiah Whitehead (right wrist) are out Thursday against the Bucks.