Robert Williams, the Celtics' top pick in the 2018 NBA draft, took responsibility Tuesday for missing a flight to Boston and the team's first summer league practice.

The 20-year-old Williams, a projected lottery pick who slipped to the Celtics at No. 27 after concerns were raised about his commitment and work ethic, spoke to reporters before practice at the Auerbach Center, the team's new practice facility in Brighton.

After attending his introductory press conference and a community event in Boston on Friday, Williams flew Texas to deal with personal business.

Williams was expected to fly back to Boston on Saturday. He moved the flight to Sunday and then missed that departure.

"Missed communication by me. It's all on me," Williams said. "So we're going to push forward and move past that."

The 6-10 big man did participate in Monday's practice, and when he arrived in Boston, the team had a message for him.

"Just accountability and just stressing that this is a job. Things are different in college so just knowing what you have and the opportunities are limited."

It wasn't Williams' first slip up since being drafted by Boston last month. After missing a conference call with the media the morning after the draft, he admitted he missed the teleconference because he overslept.

Williams said Celtics head coach Brad Stevens is among those inside the organization who have spoken to him about the incidents.

"He obviously explained to me about the opportunity. There's not that many so definitely a real wake-up call."