BOSTON – Leaning against a paint-splashed concrete wall, a man who has seen everything declared he had seen something new. Tommy Heinsohn is 82. He shared a frontcourt with Bill Russell. He coached John Havlicek. He broadcasted the majority of Larry Bird’s career. And there was Heinsohn, hours after Isaiah Thomas poured in 53 points Tuesday night and assaulted the Washington Wizards in a 129-119 Game 2 win in the Eastern Conference semifinals, shaking his head in astonishment.

“That,” Heinsohn told The Vertical, his voice trailing off. “I’ve never seen anything like that.”

This was the game Thomas was supposed to struggle in – should have struggled in. He almost didn’t play. He spent six hours Monday with an oral surgeon repairing the damage an errant Otto Porter elbow caused early in Game 1. On Tuesday, it was another four hours to treat the pain and reduce the swelling.

“[My mouth] was big,” Thomas said. “It wouldn’t have looked good on TV.”

He could have begged off because of that. This, too: The story of the tragic death of Thomas’s sister, Chyna, has been well told. Tuesday would have been Chyna’s birthday. She would have been 23. The emotion could have overwhelmed him. Instead, it fueled him further.

“The least I can do,” Thomas said, “is go out there and play for her.”

Obstacle after obstacle is placed before him, and Thomas has vowed: I will not be broken. He fought through the raw emotion of Chyna’s death to pull the Celtics out of a 2-0 hole to beat Chicago in the first round, and he has backed the Wizards into a corner in this series, outdueling John Wall in the process.

These are the moments Thomas has wanted, what he has craved. The unheralded, twice-traded kid from Tacoma, Wash., has worked himself into a two-time All-Star, a viable MVP candidate, yet these are the games he has longed for.

“I always dreamed of moments like this,” Thomas said. “Winning playoff series, winning playoff games. Close games, overtime games in the playoffs. Because those are where legends are born. And one day I want to be one of those guys.

“Until you win one of them championships, you can’t call yourself a great player.”

A great player. This is what Boston took from Phoenix for the low price of a first-round pick and a handful of spare parts. Brad Stevens wasn’t the first coach to believe in Thomas. But he was the first to truly empower him. He heard the calls to make Thomas a super sub. He built an offense around him instead. He thought he was getting a good player. He got an outstanding one.

“I knew he was special with the ball, I knew he was special in pick-and-roll,” Stevens said. “He’s improved quite a bit as a shooter. His work is very deliberate. He’s better off the dribble shooting the ball. He’s more of a threat at the 3-point line on pull-ups than maybe ever before.

“He’ll just keep getting better, because I’ve never seen him not. It’s all he does. He just keeps getting better.”

Two down, two to go, and Boston is rolling. A humiliating first-round defeat was avoided, and a trip to the conference finals is now in sight. For the second straight game the Celtics erased an early double-digit deficit. Wall’s brilliance (40 points and 13 assists) was upstaged by Thomas’ 53 – nine of which came in overtime. Indeed, the smallest player on the floor is the one who oozes the most confidence in the biggest moments.

“Havlicek had the same confidence,” Heinsohn told The Vertical. “But this guy, this little guy goes out and says, ‘Give me the ball.’ We ran things for Havlicek. This guy takes it and he just goes.”

It’s on to Washington now, with the Wizards, down 2-0 in the series, ready to make a last stand. They are too talented, too well coached to go quietly. Yet Thomas has proven to be a handful. His burning desire to be a great player is matched only by an ache to be a great Celtic. He eyeballs the pictures of Russell, Havlicek and Bird painted on the TD Garden’s walls; he gazes up at the retired numbers in the rafters regularly. He reveres Boston’s history and badly wants to be a remembered part of it.

“If he keeps playing here,” Heinsohn said, “his number will be up there.”

Thomas ran off the court on Sunday, with chants of “M-V-P!” following him into a tunnel. The Celtics are two wins from a trip to the conference finals, and Thomas will never accept that is where this run will end. With a busted mouth and a heavy heart, Thomas is coming for immortality.

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