Let’s start there. Boston and Tatum not reaching a contract extension this coming summer/fall doesn’t mean Tatum is necessarily leaving town. Far from it actually. In that somewhat unlikely scenario, the Celtics would retain the right to match any contract Tatum could sign as a restricted free agent.
At this point, the max deal Tatum could sign with another team in the summer of 2021 would be for four years and a projected $134,375,000. That’s a first-year salary of $31.25 million (which is 25% of the projected salary cap of $125 million for the 2021-22 season) with 5% raises. Tatum and/or Boston could also delay signing a contract extension and he could still sign a full four or five-year max deal with the Celtics in 2021. That deal would be for either four years and $140 million or five years and $181.25 million. Same first-year salary applies in this situation of $31.25 million, but the raises jump to 8% off the first-year salary.