Jaylen Brown is eligible for a huge contract extension this offseason. There appears to be a reason why the Boston Celtics have yet to sign him yet.

The Boston Celtics almost did the impossible this past playoff run in overcoming a 3-0 deficit. However, after forcing a Game 7 in the Eastern Conference Finals against the Miami Heat, their magic had run out and they were eliminated, ending their chances of making it to the NBA Finals for the second consecutive year.

This offseason, the team made some changes, like trading Marcus Smart to the Memphis Grizzlies and acquiring Kristaps Porzingis from the Washington Wizards.

While the Celtics did make changes, they do have a big decision to make, and that's signing Jaylen Brown to a contract extension. Brown is eligible for a five-year supermax contract extension worth up to $295 million. As of this writing, it has yet to happen.

During an appearance on ESPN's SportsCenter on July 3, senior NBA writer Brian Windhorst discussed the negotiations between Brown and the Celtics. While Windhorst did call it a "fragile" deal, he did express his belief that both sides can reach an agreement but said it might wait until the Celtics are finished with their "business."

What's possibly holding up Celtics signing Jaylen Brown to contract extension

"The sides are talking, this is obviously a very important and fragile deal," said Windhorst. "I know there's interest on both sides to get something done, and I would expect it to head in that direction. The one thing to point out if Jaylen Brown does extend on this deal, he cannot be traded for a year, for any deal at all, he's prohibited from being traded. So the Celtics probably want to make sure all their business is locked out before going into this…"

In regards to "business," Windhorst brings up the possibility of the Celtics having had talks about trading Malcolm Brogdon, whom almost had a deal with the Los Angeles Clippers, and that Grant Williams is a restricted free agent who "they may or may not be able to bring back."

Connecting some dots, Jayson Tatum is also campaigning hard to get Damian Lillard to Boston.

When asked by SportsCenter anchor Hannah Storm why he called the Brown extension "fragile," Windhorst says that there are aspects of the deal that need to be negotiated, such as his cap hit, which is eligible to count as much as 35 percent of the salary cap.

"I think it will get done, but he's eligible for up to 35-percent of the salary cap, but he doesn't have to get that," said Windhorst. "The Celtics can negotiate on that. They can negotiate on the player option. They can negotiate on several things. It's not as simple as saying 'Jaylen, here's your contract, sign it or not.' There's aspects within the deal that you've got to negotiate, and until it's done, it's not done."

This past season, Brown was nominated to the All-Star Game for the second time in his career and named to the All-NBA Second team. Through 67 regular season games, Brown averaged 26.6 points, 6.9 rebounds, and 3.5 assists, while recording a 49.1 field goal percentage, all of which were career-highs.

In the team's playoff run, Brown averaged 22.7 points, 5.6 rebounds, and 3.4 assists while shooting 49.6 percent from the field and 35.4 percent from the three-point line.

While a deal hasn't happened just yet, Windhorst believes that the two sides will put pen to paper at some point this offseason.