If Tatum continues his trajectory and adds titles, plural titles(I think he will), he will be a top 3 Celtic all-time far surpassing Pierce's career and showing he was better than Pierce from extremely early on in their careers.
If
Honestly I'm not sure the is that far away from guys like Hondo, Cousy, Cowens, McHale, etc. Bill and Larry are so far out in front it isn't worth mentioning them, but everyone else is catchable for a guy with Tatum's talent and start to his career.
He won’t catch Hondo unless he wins multiple titles and an MVP.
11x All-NBA; 8x All-Defense; 8 Championships
Was he even the best player on a title team though? Maybe 74, but that is the only one you could realistically argue. Hondo was a great player, but I feel like crediting him for 8 championships just doesn't make sense especially since I think they probably win the first 6 without him, you know since he joined a team that was reeling off championships that stopped even making the playoffs immediately upon Bill's retirement despite Hondo being in the prime of hic career. It wasn't until Cowens started to come into his own that Boston again became a contender.
Edit: And why does Tatum need a MVP. Hondo never won a MVP. His best MVP finish was 4th, finished 5th as well. In 74 he finished tied for 9th, but Cowens finished 4th which supports the fact that Cowens was the best player on that team, not Hondo. Hondo had just 4 1st Team All NBA. I think Tatum gets his 1st this year and he should be expected to continue to get them for the foreseeable future.
Also, to be clear, Tatum clearly needs to play more seasons to have careers anywhere close to Hondo, Cowens, Pierce, etc., I was more talking about reasonable projections (even with almost no more growth) that he is going to start being in the discussion because he is clearly more talented than those guys. But if you told me I could have Tatum this season or any season in Hondo's career, I'd probably take Tatum in that 1 season scenario right now.