Amari Williams expected to be playing Friday night — but for the Maine Celtics, not their NBA parent club.

But after starting center Neemias Queta came down with an illness before Boston’s game in Brooklyn, the team called up Williams for extra frontcourt insurance. And after Queta and his backup, Luka Garza, both fouled out, head coach Joe Mazzulla turned to the rookie two-way player to help the Celtics secure one of their wildest wins of the season.

Williams, after sitting for all of regulation and nearly the entire first overtime session, subbed in with 7.9 seconds left, received a long-range inbounds pass from Sam Hauser and fed Payton Pritchard for a quick 3-pointer.

After fellow rookie Hugo Gonzalez hit a last-second corner three on the next Celtics possession to force a second OT, Williams played the rest of the way. He converted a go-ahead and-one layup — just the second made field goal of his young NBA career — and later punctuated Boston’s 130-126 victory by swatting away a shot from Nets guard Nolan Traore with 40.6 seconds to play.

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The late block capped a whirlwind day for the second-round draft pick.

“Funny enough, this game, I flew in two hours before from Maine,” Williams told reporters postgame, via CLNS Media. “I did the shootaround this morning, thought I was going to play there tonight, and then Neemi wasn’t feeling great, so I flew in today. Just kind of being ready for whatever. You don’t really know until you get here if you’re going to suit up or not, so just kind of being ready for whatever.”

Drafted 46th overall out of Kentucky last summer, Williams has logged just 52 minutes for Boston this season, most of those coming late in blowouts. But he’s posted encouraging numbers in the G League, averaging 17.3 points, 11.7 rebounds, 4.4 assists and 1.8 blocks since Maine’s regular season began.

Mazzulla praised the 23-year-old for his readiness.

“For Amari to be able to sit through the entire game and be ready and execute some of the plays that we’ve gone over in practice, it’s a credit to him and the coaching staff,” the Celtics coach told reporters in Brooklyn. “Those are the guys that work with him, getting him ready to go, and say the same thing for Hugo’s mindset. Those guys made big plays, and the (player development) staff gets those guys ready to go every day.”