But there he was, sliding down to the Boston Celtics at the 27th pick in the NBA draft Thursday night.

“It’s not my job to know what the other 26 teams are thinking,” Celtics coach Brad Stevens said after adding the two-time Southeastern Conference defensive player of the year. “We thought that he was the right pick for us, and we felt very fortunate to be able to get him.”

A 6-foot-10, 237-pound Oil City, Louisiana, native, Williams was considered a certain first-round pick as a freshman — maybe in the top 10.

But he decided he wasn’t ready. He returned and averaged 10.4 points and an SEC-leading 9.2 rebounds while leading the Aggies to the NCAA tournament’s Sweet 16.

“He’s a rim-protector and a rebounder, and he can play above the rim on both ends of the court,” Celtics general manager Danny Ainge said. “We don’t have much of that.”

Williams, 20, still has some work to do— especially on offense, Ainge conceded.

“He’s still young,” Ainge said. “He brings an upside. There’s still parts of his game that are untapped. But rim-protection and rebounding, we know that.”

Stevens said current Celtics big men Al Horford and Gordon Hayward will be a big help in getting Williams ready. “He won’t have any better role models than the guys in front of him,” the coach said.

The Celtics followed an appearance in the 2017 Eastern Conference finals with an offseason overhaul that brought in Hayward and Kyrie Irving. Despite losing both of them to injuries during the season, they returned to the league’s final four thanks in part to Jaylen Brown and Jayson Tatum, back-to-back No. 3 picks in the previous two years.

But after stockpiling eight first-round picks in the previous four drafts, including three lottery picks, the Celtics were left with just their own first-rounder this year and no picks in the second. They could have up to four first-rounders next year.

Ainge said he looked into moving up to the top of the draft.

“We spent the last couple of weeks figuring out what the cost was. We knew that was very, very unlikely,” he said.

Acquiring a second-round pick didn’t interest him all that much.

“We don’t need more young players really,” he said. “We feel good about adding one that does something that we don’t have.”

Speaking at the team’s brand new practice facility, the Auerbach Center, Stevens said Williams was a good athlete with good feet and an arm-span of 7-feet, 5-inches. Williams blocked 155 shots in college — third in Texas A&M history, with 78 as a sophomore that were the second-most in a single season for an Aggie.

“Right now he is an elite athlete, with incredible length,” Stevens said. “And he plays well above the rim. Obviously, if you can have four shooters in the game and a guy like that rolling to the rim, you can just throw it into the air and he can go get it.”

Because Williams was expected to go higher in the draft, the Celtics did not have a workout for him in Boston. Stevens said having a prospect visit is “just a formality in a lot of ways” for Ainge and his staff.

“They’ve already watched them pay live a bunch, spent time with their coaches and everybody else,” Stevens said. “But I don’t think you need to watch must to tell how athletic he is. It’s pretty obvious.”