BOSTON – The wait is finally over: Avery Bradley will make his season debut tonight against the Memphis Grizzlies.

Mark the date on your calendar, because it could be a key one in the Boston Celtics’ 2012-13 season.

The Celtics and their fan base have been patiently waiting for this day since May 18, 2012, when Bradley began his path toward offseason double-shoulder surgery. That’s the last time Bradley participated in an NBA game and harassed an opponent with his ferocious on-ball defense.

Opposing teams, and particularly their top perimeter scoring threats, have headed into games against the Celtics with a smile on their faces ever since. Ask Dwyane Wade, Jrue Holiday, Tony Parker, Stephen Curry or any other scoring guard in the league if it’s easier to get out of bed in the morning knowing that they won’t have to face off with Bradley that night. They’ll all answer with a resounding, “Yes!”

Bradley’s defense changes the game dramatically. It’s disruptive enough to alter opposing players’ mental state. (Yes, we’re talking about you, Jameer Nelson). Bradley possesses a skill set that can single-handedly change a game at the defensive end of the floor.

There are only a handful of guards in the league who can make a claim to that type of skill, and one of them happens to be on the team Boston plays against tonight. Tony Allen, who became known as a premier defender here in Boston, is essentially Memphis’ version of Bradley.

“Tony is a guy – we just put him on the guy we think is the best scorer on the other team on the perimeter and we just don’t think about it anymore,” Grizzlies head coach Lionel Hollins said Wednesday morning.

That’s what Doc Rivers and the Celtics had last season when Bradley was on the floor. Without those skills in their toolbox, the C’s have fallen back dramatically at the defensive end and stumbled out to a 14-16 record.

The Grizzlies are currently 19-9, but where would they be if Allen missed the first two months of this season? Hollins told Celtics.com on Wednesday what he thought about that hypothetical.

“It would be a problem,” he said bluntly. “We wouldn’t want to do that.”

Neither did the Celtics with Bradley, but they were forced to do so for 30 games. Their defense, which was quite possibly the best defense the NBA has ever seen while Bradley emerged during the final quarter of last season, has been below average this season. Boston ranks just 17th in opponent scoring and 19th in opponent field goal percentage.

Those rankings are likely to experience a steady incline over the final 52 games of this season. There’s plenty of evidence to make us believe that such will be the case.

Bradley started at shooting guard alongside Rajon Rondo, Paul Pierce, Brandon Bass and Kevin Garnett in 15 of the Celtics’ 66 regular season games last season. Opponents didn’t just struggle in those games. They were absolutely terrible. Opposing teams shot just 39.4 percent from the field and scored an anemic 85.4 points per game in those contests. Oh, and by the way, the Celtics went 12-3 in those games.

Let’s take note of this caution flag: those numbers aren’t going to return overnight for the Celtics now that Bradley is back in the fold. Anyone who knows anything about NBA basketball knows that the C’s are not on the verge of greatness right now.

Rather than expecting wholesale changes overnight, the Celtics and Bradley are taking a realistic approach to his return. They just want to see steady improvement, both from him and for the team as a whole, while avoiding any worries about unreasonable expectations outside the locker room.

“I try not to worry about it,” Bradley said of outside expectations surrounding his return. “Like I said, all you can do is go out there and play hard. That’s what I’m going to do – that’s all I know. That’s how I play. Just go out there and leave everything on the floor and [tonight] that’s what you guys will see.”

Bradley’s game isn’t based on canning jumpers in an opponent’s face or crossing someone up on the perimeter. He’s just going to play harder than you while harassing you. Plain and simple.

That’s how Bradley made his name last season while playing a key role in the Celtics’ dramatic turnaround. That’s why today’s return could easily be considered the most important single day of Boston’s season to date.