Sources: Marcus Smart, Celtics don't reach extension agreement before deadline

The Boston Celtics and guard Marcus Smart’s agent remained fully engaged until the contract extension deadline of 6 p.m. ET Monday, but could not bridge the gap for an agreement, league sources told The Vertical.

For the Celtics, luxury-tax implications moving forward were a significant hurdle in completing Smart’s extension, league sources told The Vertical.

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Smart will be a top guard option in restricted free agency next summer, and his agent, Happy Walters, had a similar experience to Smart’s in 2014, when Jimmy Butler and the Chicago Bulls were relatively close in extension talks but could not reach an agreement. Butler became a maximum-salary player the following summer.

Smart, 23, entered training camp this season in the best shape of his professional career and with an improved jump shot. After the Celtics revamped the roster with the additions of Kyrie Irving and Gordon Hayward, Smart has received praise from Celtics coaches and players for his leadership and acceptance of a reserve role. Smart has often defended the top perimeter guards for the Celtics.

He finished the preseason averaging 10.3 points, 3.8 rebounds, two assists and 58.3 percent 3-point shooting in four games.

For Boston, allowing Smart to reach restricted free agency next offseason brings the threat of a rival team creating an offer sheet that would make things difficult for the Celtics because of their high payroll. Teams are expected to value Smart’s abilities on both ends of the floor, as well as his pedigree as part of a winning culture in Boston.

Smart was the No. 6 overall pick in 2014 by the Celtics. He averaged 10.6 points, 4.6 assists, 3.9 rebounds and 1.6 steals in 79 games last season.