LAS VEGAS — Celtics forward Al Horford has been stealing glances of the team’s summer league games when he is able, and he has been especially intrigued by rookie forward Jayson Tatum.

“I’ve been impressed with his ability to just score on isolations,’’ Horford said in a telephone interview. “Coming into the NBA — you can do it at a high school or college level — but the fact that he is so comfortable in those positions, I’m very impressed by it. And, I mean, he’s only 19 years old, and he’s going to keep getting better, so I was very impressed with him, and the way he can shoot the ball.’’

Tatum, the third overall pick of last month’s draft, averaged 18.7 points and 9.7 rebounds in three games at the Utah Jazz summer league last week. Horford also said second-year forward Jaylen Brown appears to be playing with increased confidence and energy, and that his improvements are evident.

Last season Horford was the prize of the summer for the Celtics. Boston signed the forward to a four-year, $113 million deal, yet aside from Brown being drafted, the roster mostly maintained its form from the previous year.

But this offseason has been a whirlwind, due in large part to the impending signing of another All-Star free agent, Gordon Hayward, whose four-year, $128 million deal made several other moves necessary to clear salary cap space.

Now, suddenly, Horford will be one of the more tenured Celtics when he returns for just his second season. When he considers the additions that have been made to a 53-win team, the possibilities for next year seem unending, he said.

“I think we have a chance to be really good,’’ Horford said. “I think the key for us is to make sure that we keep working, that we keep getting better. The biggest thing is once we get together to really start working on the chemistry, really getting comfortable playing with each other.’’

In addition to Hayward and Tatum, the Celtics added forward Marcus Morris in Friday’s trade that sent Avery Bradley to the Pistons. Boston is rapidly collecting tall, versatile players who are capable of drilling 3-pointers.

“We have guys that can shoot the ball that kind of do it all on the court,’’ Horford said. “The way basketball is being played, you have to be able to do a little bit of everything, and those guys are able to do a little bit of everything. So it makes us that much tougher as a team.’’

Hayward is the most important new piece, of course. The forward is expected to have his introductory press conference at the team’s Waltham training facility early next week. Horford thinks the addition of Hayward should be a boon.

“I’m very excited about Gordon,’’ he said. “I believe that offensively he can really shoot the ball. He can finish very well at the rim, and he’s going to be a threat for us in transition, attacking. He has a bigger body, so he’ll be able to rebound some, and defensively he can hold his own. Playing with a guy like that is going to be a lot of fun and I can’t wait for us to get to practice and all of us start to build that chemistry.’’

Each new arrival means there is also a new departure. Because of salary cap limitations, Boston had no choice but to relinquish one of its key pieces to free up space to sign Hayward. President of basketball operations Danny Ainge ultimately made the difficult choice to part with Bradley, in part because he will be an unrestricted free agent after next season.

Horford said that Bradley’s absence will be significant.

“He’s one of the guys I really leaned on a lot this year that was always encouraging me and positive,’’ Horford said. “So he’s somebody that I have a lot of respect for and I wish him the best. Detroit is really lucky to have him.’’

After the Celtics lost to the Cavaliers in the Eastern Conference finals, Ainge said that Horford had been playing through nagging elbow pain since suffering a sprain in March. The forward says he feels better now, and is looking forward to starting this possibility-filled season.

“I’m doing good,’’ Horford said. “The time off always helps. I’m doing well, just working. I’m trying to get better, get stronger.’’

.   .   .

The Celtics’ summer league game against the Lakers here on Saturday was sold out on Friday afternoon, an extreme rarity. The game was billed as a battle between the No. 2 pick in this year’s draft, Lakers point guard Lonzo Ball, and the No. 3 choice, Celtics forward Jayson Tatum. And, of course, it was Celtics vs. Lakers.

Ball wowed the Lakers-heavy crowd with his dazzling passes, but Tatum had the final say, tallying 27 points and 11 rebounds as the Celtics took an 86-81 win.

Ball finished with 11 points, 11 rebounds and 11 assists for the Lakers.

Brandon Ingram, the No. 2 overall pick of the 2016 draft, missed the game because of leg cramps suffered in the Lakers’ game on Friday and did not play.

The Celtics started the game without a true point guard, and it became obvious. Nearly four minutes passed until Boston scored a point.

The Celtics got a big lift from second-round draft pick Semi Ojeleye, who made five 3-pointers and scored 19 points.

Point guard Demetrius Jackson, who missed the last two games as the Celtics sought trades to free up salary cap space to sign Gordon Hayward, returned and had 6 points, 5 rebounds and 4 assists.

After trailing, 52-41, midway through the third quarter, the Celtics stormed back and closed the period with a 22-5 run that was capped by a pull-up 3-pointer by Tatum at the buzzer. The Celtics led, 72-67, before Lakers forward Kyle Kuzma hit a three and Ball converted an acrobatic layup to tie the score with 2:56 left.

After a timeout, the Celtics answered. Tatum attacked the rim for a layup and Ojeleye hit a trey. Boston never trailed again.

Adam Himmelsbach can be reached at adam.himmelsbach@globe.com. Follow him on Twitter @adamhimmelsbach