Tatum will now be eligible for restricted free agency in 2021 assuming he turns down a $12.9 million qualifying offer.
Nobody summed up the Celtics' 2018-19 season more than Tatum.
After exceeding expectations as a rookie, the 21-year-old was supposed to take a big leap as a sophomore. Instead, his scoring efficiency went backward. He shot 45.0 percent from the field and 37.3 percent from three-point range, down from 47.5 and 43.4 percent, respectively.
ESPN's Tim Bontemps noted Tatum began attempting more mid-range jumpers, an approach he likely modeled after Kobe Bryant. He has worked out with Bryant and looked to the Los Angeles Lakers legend as an inspiration for his game.
Drew Hanlen, Tatum's trainer, told Bontemps he talked to the Celtics forward about tweaking his offensive mindset.
"I basically said, 'If you just shoot six free throws per game and six 3s per game at the same percentages you did last season, then you'll raise your [scoring] average from 15.7 to 20.7,'" Hanlen said.
The departure of Kyrie Irving may help Tatum as well. Irving acknowledged in January he had struggled to fully adapt to the role of an on-court leader. ESPN's Jackie MacMullan also provided a peak behind the curtain as to the off-court issues that held the Celtics back, which included a general disconnect between Irving and the Celtics' younger players.
Triggering Tatum's fourth-year option was a no-brainer for Boston. His next contract could be similarly straightforward if he rebounds in 2019-20 to firmly establish himself as the Celtics' No. 1 scoring option.
Should Tatum continue to underwhelm, however, the front office could have a difficult decision on its hands, not unlike ongoing extension negotiations with Jaylen Brown.