Wednesday was one of those nights when we’re reminded Avery Bradley is 22 years old and in his third year in the NBA.

As one of the best on-ball defenders in the NBA, his importance to the Celtics is unquestioned, and Doc Rivers was banking that he would provide resistance to Brooklyn guard Deron Williams, who when healthy is one of the league’s best because he can shoot from the perimeter and bounce off defenders without a dent.

When Bradley attempted to defend Williams, the blame for whatever contact ensued was assigned to Bradley, who picked up two fouls in the first 3:44, and then his third with four minutes left in the half.

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Thirty-nine seconds into the second half, Bradley, eager for another opportunity against Williams, was whistled for his fourth foul when the two collided and their feet tangled, causing Williams to tumble.

That call infuriated Bradley. He stood there, arms raised, looking at official Sean Corbin with an expression of helplessness. It was a rare occasion when Bradley was taken out of his game defensively, losing his edge because his aggressive defense was hindered by tight officiating.

After his fourth foul, Bradley sat near the end of the bench to cool off, putting his warm-up back on, a sign that his respite would be extended. He was confused and irritated. Veteran teammate Jason Terry attempted to calm him down by offering pointers on how to defend Williams.

It was the first time this season Bradley and Williams faced off because Bradley missed all three previous meetings recovering from surgery on both shoulders.

The Celtics lost, 101-93, and the game was not that close. Williams finished with 29 points and 12 assists, setting the tone for the Nets’ offense. His dribble penetration freed up teammates for open shots, and the Celtics’ defense crumbled.

Bradley’s defensive prowess is no longer a secret. Opposing teams have figured out two ways to unnerve him: pressure him as much as he pressures them or use his aggression to get him into foul trouble. The Nets went with the latter as Williams knew the key to breaking down the Celtics was to eliminate Bradley.

“Avery’s a tough defender and you can’t really afford to play around with the ball, so I was just trying to make quick moves,” Williams said. “And once he was in foul trouble, I was trying to be more aggressive. [I was] really comfortable for the most part and I was able to attack the rim, get to the free throw line 10 times, that helps build confidence.”

Bradley felt partially responsible for Williams’s performance, seemingly embarrassed that the opposing point guard was able to dominate the game. The Celtics shot 45.7 percent and were competitive on the boards (37 rebounds to the Nets’ 39), but their inability to contain Williams was critical.

If the Celtics are going to compete in a first-round series with, mostly likely, the Knicks, Bradley has to be a shutdown defender, forcing teams to waste valuable time getting into their offense. Bradley on the bench means Courtney Lee becomes the Celtics’ best on-ball defender in the backcourt. He is improving but is not at Bradley’s level.

After the game, Bradley was visibly disappointed because he takes such losses personally. He was angry after the Cavaliers’ Kyrie Irving scored 40 points against the Celtics Feb. 22. Bradley takes pride in making something as simple as getting the ball over half court in eight seconds a challenge.

But he provided no challenge to Williams because he was on the bench for 75 percent of the game.

“It was just hard to get going,” Bradley said. “I couldn’t get out of the gate. Really didn’t get a feel for anything at either end of the floor. It happens. All I can do is continue to keep fighting. I was frustrated throughout the game, but it happens.”

Rivers has to keep finding ways to infuse confidence in Bradley, and Bradley has to tone down the aggression when games are being closely officiated. On Wednesday, Bradley played like an inexperienced player. And he’s so critical to the Celtics’ defense that his absence proved fatal.

“He just picked up the fouls, and then, I don’t know,” said Rivers. “I actually think he’s playing really good defense, and I don’t know, those are the [calls] I didn’t like. And he just had one of those games when you pick up two fouls, you take him out, you come on the floor, and you bump into your third foul. Those games happen. It had to be unbelievably frustrating for him. It was probably really frustrating for our team as well, because we needed him.

“It’s no coincidence that right when Avery went off the floor, Deron Williams got ultra-aggressive. And I give Deron a lot of credit. When Avery picked up that first foul, he attacked him. Because they know, they want him off the floor. So, Deron’s smart. He wants to score. And the best way to do that is taking Avery off the floor.”

It’s difficult to believe Bradley has become this significant this quickly, but he has, and teams are reacting by using his aggression against him. It’s Bradley’s turn to make the next move before the playoffs begin.