Boston Celtics first-round draft pick Robert Williams, who already had overslept an introductory conference call the day after the NBA draft, missed a flight to Boston and was not present for the start of the team's summer league practices on Sunday.

Concerns about Williams' work ethic contributed to him sliding out of the lottery and to Boston at No. 27 in last month's draft. The Celtics introduced the 20-year-old big man at a news conference Friday morning, but he did not get back to town in time for Sunday's afternoon workout.

"We expected him to be here, but obviously he missed his flight," said Celtics assistant coach Jay Larranaga, who will guide the summer squad in Las Vegas. "We're excited for him to get here [Monday] and get started.

"I think everyone's disappointed. You want to get everything off to a good start in the first day, but we'll handle that internally and move forward."

Williams, who spurned an offer to be at the draft in Brooklyn to instead gather with family and friends at a Buffalo Wild Wings in his native Louisiana on draft night, admitted he overslept a conference call the next morning. A rescheduled call an hour later was plagued by poor sound quality.

Williams arrived in Boston last week and met with coach Brad Stevens, who reminded the athletic forward that, in order to reach his potential, he would have to work hard. Williams also chatted with All-Star forward Al Horford and later noted he plans to use the veteran big man as a guide.

At Friday's news conference, Williams referenced hard work at least seven times during a six-minute introductory Q&A with Boston reporters.

But his absence Sunday did nothing to dispel the lingering notion that Williams is not committed to putting in the work necessary to emerge as a contributor on a Celtics team with championship aspirations.

"I think with every rookie that comes into the NBA it's really a race to maturity, and some people are further ahead like a Jayson Tatum," Larranaga said after Tatum practiced with the summer leaguers on Sunday. Tatum will not participate in summer league, but Stevens asked him to join the Celtics' youngest players for practices before they head west.

Robert Williams slipped in this year's draft in part because of concerns about his work ethic. Nelson Chenault-USA TODAY Sports

"A lot of guys have some work catching up to do," Larranaga added. "So, Robert, like I said, when he gets here [Monday] we'll start working with him. And I'm just excited to get to meet him."

Stevens told ESPN earlier this week that Williams must be ready to work in order to carve out a role with this team.

"I think he's got a chance to be a good player. As I told him when we met, there are a lot of guys who have a chance to be a good player. So it's all about how you work," Stevens told ESPN.

"It's all about how you embrace these opportunities, and he can't have better examples than the guys down in that locker room to follow. So it's on him to follow. And we're going to invest a lot of time and energy into him."

Stevens later noted, "I think timeliness is important and, you know, if that's the last time then that's good for all of us."

Alas, it was just a matter of days before Williams was late again, and he's going to have to work to win the trust of the organization early in his NBA career.

Asked about the criticism regarding his work ethic, Williams said Friday: "People question my motor a lot, but I work hard and know I can work hard. I'm a hard worker. Just being in this organization, knowing the morals of this organization and knowing what it takes to be in this league, definitely has me [excited]. I'm ready to start."