Little-used Celtic Aaron Nesmith has stayed prepared for anything, and that meant playing time in Game 1

Aaron Nesmith (right) blocked three shots in Game 1, including this attempt by Miami's Tyler Herro.Matthew J Lee/Globe staff

MIAMI — After a recent Celtics practice at the Auerbach Center in Brighton, a full-court scrimmage sprouted involving players who haven’t done much full-court anything in recent months. The lineups mostly included former G League players who are happy simply to be barnacles on the team’s sudden playoff run: Malik Fitts, Brodric Thomas, Matt Ryan, Juwan Morgan, Luke Kornet.

And then there was Aaron Nesmith, a lottery pick in the 2020 NBA Draft. As a rookie one year ago, Nesmith used his hustle, athleticism, and defense to carve out a key role late in the season. He then shined at the Las Vegas Summer League and entered this year with high hopes.

But instead of a continued ascension, this has turned into a lost season for the second-year guard. He played in 52 games, averaging 11 minutes, and he has not played meaningful rotation minutes since February.

As the Celtics climbed toward the top of the Eastern Conference, this promising young player was essentially left behind. But he insists that it has not dented his confidence, and the Celtics coaches insist that Nesmith has not regressed. The rotation has just been condensed, and he has not been a part of it.

“Look, it’s easy to go down the path of ‘this is happening to me, I can’t believe this is happening,’ and feeling bad about it,” said Celtics assistant Joe Mazzulla, Nesmith’s position coach. “And there’s another path of understanding what’s going on around you and just maintaining your professionalism and being ready, because at the end of the day, everyone gets an opportunity.

“I always tell him, ‘You’ll be judged more on how you handle the opportunity when you get it. If you handle it with professionalism and execute your role, you’ll be really appreciated for that. If you don’t, it’ll look bad, because you had so much time to prepare and get ready, and you didn’t do it.’ ”

Mazzulla said those chances can arrive suddenly, and Nesmith understood. Then prior to Game 1 of the Eastern Conference finals against the Heat Tuesday, guard Marcus Smart was ruled out because of a foot sprain and Al Horford was sidelined because of COVID-19 protocols.

Nesmith went from playing pickup games with third-stringers to being thrown into his first legitimate playoff action this year, on a massive stage.

His 11 minutes of action were not perfect. He missed all three of his 3-point attempts and fired one pass deep into the stands. But he left no questions about his effort level, and he made two of the Celtics’ most spectacular defensive plays of this postseason.

Aaron Nesmith played 11 minutes in Tuesday's Game 1 loss to the Heat.Lynne Sladky/Associated Press

The first came moments after he checked in at the start of the second quarter, when Miami blitzed upcourt on a four-on-one that most defenders would simply cede. Nesmith stood his ground and swallowed up Caleb Martin’s dunk attempt.

Then in the third quarter, the Heat had a two-on-one break before Nesmith timed his jump perfectly and pinned Victor Oladipo’s layup off the glass. Miami’s Jimmy Butler grabbed the loose ball and had his shot swatted away by Nesmith, too, but a foul was called on the play. Nesmith was furious with himself for not closing out the sequence with perfection.

“None of that surprises me from Aaron,” Celtics center Robert Williams said. “One of the hardest-working guys we’ve got on this team. One of the guys that is always staying ready.

AARON NESMITH REJECTION 🚫🤯 pic.twitter.com/l4jhJ4JQ0z

— Celtics on NBC Sports Boston (@NBCSCeltics) May 18, 2022

“But I’m glad he’s learning how to take advantage of any opportunity that he gets. I just told him that whether you get in for two minutes or 20, just try to be a player for us. And it’s not just offensively. You see the chase-down blocks; it’s the defensive effort.”

Nesmith finished with three blocks and a steal. The Celtics played the Heat evenly during his 11 minutes on the court and were outscored by 11 points during the other 37.

Although Nesmith’s hustle and defense have been pleasant surprises, he was drafted out of Vanderbilt in large part because of his potential as a 3-point shooter. He connected on 27 percent of his shots this season but had just 115 attempts, a sample size that the Celtics believe is too small to be judged.

When Nesmith played, he generally got one or two shots from long range, and no real time to establish a rhythm. Still, 27 percent is not good enough, and he knows that.

“I’ve just got to shoot the ball better than I did this season,” he said. “I just want to make the other guys better when I’m out there.”

Nesmith’s play on Tuesday might have been enough to earn him another chance, or Smart might return next game and keep Nesmith on the bench for the rest of these playoffs. The Celtics are confident that Nesmith will remain prepared regardless, and their belief in his potential has not wavered.

“I think the most important thing is trying to maintain perspective,” Mazzulla said. “He just needs to understand where he’s at and what’s going on around him. Some of it he can control and some of it he can’t.”