Sean Deveney of Heavy.com reported "the chance is pretty slim" the two sides come to an agreement before an Oct. 21 deadline. Deveney explained Brown is slated to be a restricted free agent following the 2019-20 season, so Boston would have the chance to match any offer that came his way next offseason.
This is nothing new for the Celtics.
Deveney listed Jared Sullinger, Kelly Olynyk and Marcus Smart as players Boston could have signed to extensions following their third season in the league, and the team elected against it every time. Sullinger and Olynyk eventually signed elsewhere, while Smart signed a deal that was less than he was originally hoping for after his third year.
Brown is also a trickier case because he has a higher ceiling than all of those players but hasn't yet consistently produced like someone worthy of a max contract.
Boston selected Brown with the No. 3 overall pick in the 2016 NBA draft, and he averaged a modest 6.6 points per game as a rookie and rotational player. His second season in the league was his best at 14.5 points, 4.9 rebounds and 1.0 steals a night behind 39.5 percent shooting from three-point range, but his numbers dropped nearly across the board in his third.
He posted 13.0 points, 4.2 rebounds and 0.9 steals per game last season and shot 34.4 percent from deep.
However, Brown looked like a blossoming superstar during the 2018 NBA playoffs when Kyrie Irving and Gordon Hayward were sidelined with injuries. He spearheaded a run to Game 7 of the Eastern Conference Finals alongside players such as Jayson Tatum and Terry Rozier and poured in 18.0 points a night.
A versatile playmaker who can shoot from the outside and attack the rim off the bounce like he did in those playoffs is sure to draw interest from around the league, especially since he is only 22 years old. How he fits in alongside Kemba Walker and plays this season will likely go a long way toward determining the offers he receives as a restricted free agent.
Boston will then have to make a decision about his long-term future.