BOSTON – Two weeks ago, Celtics rookie Robert Williams was on his way to Portland, Maine, for what was expected to be an extended run with the G-League Red Claws so he could get some minutes outside of the crowded Boston frontcourt.

Four injuries to members of that frontcourt later, Williams was on the floor for crunch-time minutes Wednesday night in what was ultimately a failed comeback bid against the Phoenix Suns at TD Garden.

While Williams acquitted himself quite well in the time with eight points, eight rebounds and five blocked shots in 24 minutes off the bench, the reliance on the 21-year-old rookie was not sustainable as the Suns struck for 21 offensive rebounds in a 111-103 stunner.

“It breaks people’s spirit,” Kyrie Irving said after the second straight setback. “(Giving up) offensive rebounds, getting new possessions, second-chance points.”

Not that Irving was blaming Williams. Quite the contrary, he turned his criticism on the rest of the team for not realizing in the moment that Williams is not a seasoned veteran like an Aron Baynes, an Al Horford, or a Marcus Morris. While Williams has showed he is capable of making the spectacular play on defense – blocking Anthony Davis twice in a game two weeks ago and twice now getting five blocks a game – that aggressiveness can leave him out of position to finish off the defensive possession.

“With Rob, I think that he’s 21 years old,” Irving said. “We expect a lot out of Rob. But also there’s a learning curve for him. He does a lot of great things already. I think the sky’s the limit for his potential and what he brings to our team – obviously, getting more repetitions at this point, creating a lot of lob opportunities at the rim, as well as getting blocks and protecting the paint.

“But us, as guards, when we’re missing guys we have to help Rob rebounding as well. He’s contesting and going for every block. Our weak side, and crashing in, and helping him out, is just as important as him rebounding. So we, as a team, have to cover for each other.”

That is something the Celtics did not do enough with Horford (patellofemoral pain syndrome), Morris (knee tendinitis) and Guerschon Yabusele (sprained ankle) out of action to start Wednesday’s game, and then losing Baynes to a fractured fourth left metacarpal in the first two minutes of the night. With Baynes now expected to miss about a month after having surgery on his hand, Horford ruled out and Morris questionable for Friday night’s showdown vs. the Milwaukee Bucks, the Celtics will have to go back to the type of team rebounding and help defense they used before the arrival of Baynes and Morris last season.

“It’s a real concern,” said Celtics coach Brad Stevens, who should at least have Yabusele back after he missed the past five games.

Williams is doing his best to remain grounded while being thrust into the fire.

“Not a good sight for us knowing how much we need Baynes,” he said. “But next-man-up mentality. You’ve just got to be ready.”

While his extreme athleticism and leaping ability allow Williams the ability to contest shots from the paint all the way out past the 3-point line on a switch, he said he knows going for the block isn’t always the prudent choice on every possession.

“That’s just something that I need to work on defensively,” he said. “When to go, when to stunt, when to stay with my man. So, in due time, I’ll learn it.”

In the meantime, the rookie will likely have to learn on the fly during a stretch of the team’s highest-profile games of the regular season. It’s a study session that injuries have forced to speed up considerably over the past two weeks, but one in which Williams has done an admirable job of keeping up with to this point.

“I think he’s showed himself well these first couple of months of the season,” Stevens said, “from where what maybe I would have expected from the very beginning of the year, and certainly in the summer. So that’s encouraging.”