On Friday, the Celtics held their final practice of the training camp season, as their opening night game against the 76ers at TD Garden is just five days away.

More important than anything happening on the court at the Auerbach Center in Brighton, MA was the status of forward Jaylen Brown, who exited Wednesday night’s preseason finale in the first quarter with left hamstring tightness.

“He was able to do everything but the live stuff,” Mazzulla said when asked how Brown fared during the final practice of the training camp season.

When asked how Brown felt after practice, Mazzulla said, “I haven’t caught up with him yet. I mean, I expect him to be at his best every single day and we’ll kind of take it from there. But one day at a time.”

Mazzulla went on to say Brown’s injury is a “day-to-day thing, understanding [where he’s at] day-to-day.”

At the 4:51 mark in the first quarter on Wednesday, Brown came off the floor in what looked like a routine substitution after what was a great start to the game for the former Finals MVP (7 points in 7 minutes with 2 rebounds and a steal).

But when you slow down the telecast, you can see Brown walking by the scorer’s table as head coach Mazzulla was mid-berating a ref.

Brown was grabbing his left hamstring, and said something to lead assistant Sam Cassell as he made his way down the bench.

After being seen talking to trainers by the bench, Brown headed down the tunnel back to the locker room. And a mere moments later, the Celtics announced the 28-year-old was doubtful to return with “left hamstring tightness.”

Doubtful turned to definite, as Brown’s night was over after only 7 minutes of action. And no, this is not one of those “I get it, it’s preseason” moments that I’ve been including as caveats throughout everything I’ve written this exhibition season. Whatever it was that happened, it was bad enough to end his night.

Sure, it could end up being nothing. But make no mistake about it - this year’s version of the Celtics cannot afford it to be anything more than nothing.

I went back on my DVR to see what led to the grabbing of the hamstring:

At 5:15 in the first quarter, Ochai Agbaji missed a corner three, leading to a rebound by Celtics center Luka Garza. In an attempt to push the pace, Garza got the ball to Brown, who was looking to facilitate some fastbreak points by dribbling up the middle of the floor.

After just two dribbles, Brown tossed the ball to a cutting Anfernee Simons on the left wing. And as his new teammate dribbled toward the basket in search of a streaking Garza towards the rim, Brown faded off towards the sideline while grabbing his hammy.

Simons’ pass eventually led to a turnover, which had Toronto attempting a fastbreak of their own. And as Toronto’s RJ Barrett finished a layup, Brown could be loafing back in transition, hanging by the arc while still grabbing that leg.

On the ensuing Boston possession, Brown hobbled toward the left corner while the offense began setting up some action on the right wing. Before they could get anything going, Garza was called for a questionable offensive after unsuccessfully trying to screen the Raptors’ Immanuel Quickly.

(to come full circle - this was why Mazzulla was berating the ref)

With the stoppage in action, subs came in, sending Brown to the bench. His night was over, and Celtics fans all collectively began to hold their breath.

It appears they can let out a minor sigh of relief based on Mazzulla’s comments today after practice. But his minutes, usage and production in the early part of the season will all be viewed through this prism, especially if Brown struggles out of the chute.

If there’s one thing this Boston team cannot afford this season, it’s losing Jaylen Brown for any sort of extended stretch.

With Jayson Tatum likely out for the entire season as he continues to rehab his Achilles, this has to be Brown’s season to shine. The second-banana spot must be passed along to someone else, as the top spot unequivocally goes to the 10-year pro out of Cal.

Tatum is out. Al Horford plays for the Warriors. Jrue Holiday played for the Trailblazers. Kristaps Porzingis plays for the Hawks. Luke Kornet plays for the Spurs.

The early part of this season is going to be a lot of this team figuring out who they are moving forward. Brown was-and-is obviously going to be a huge part of that. Missing time at the beginning of the season as the new foundation is being laid down would be a devastating blow towards Boston’s chances of remaining a top team in the Eastern Conference sans Tatum.

For the sake of having a competitive basketball season, let’s hope Mazzulla’s postgame read on the situation was an accurate one.

…but if Patriots All-Pro cornerback Christian Gonzalez taught us anything about hamstrings this year, they can make for a tricky, unpredictable rehab process.

Tom Carroll is a contributor for Roundtable, with boots-on-the-ground coverage of all things Boston sports. He's a senior digital content producer for WEEI.com, and a native of Lincoln, RI.