He has played for four teams in his NBA career. I am sure thos coaches also tried to shore up his defensive deficiencies.
I'm not optimistic about Kanter's defense, but Stotts was able to mostly hide him through two rounds against good teams (OKC and Denver). Winning two playoff series in the West is nothing to sneeze at and not a fluke, especially with Portland, which isn't some kind of amazing juggernaut (full respect to Dame and CJ, who are great).
Kanter is sort of kind of playable as purely a drop defender, which means that against teams without dynamic guards who are threats to pull from 3 off screens, he's playable. GSW completely toasted him with off-ball action and Steph, but mostly Steph. The math of exploiting drop defense without that kind of guy is just really hard--you end up needing really elite mid-range guys, and even then, you're on the knife's edge in terms of 2*X not being over 1.00 PPP enough of the time (see Irving, Kyrie, games 2-5).
Where things get interesting is that neither of the top 2 teams in the East have anyone threatening in PnR actions in terms of shooting (maybe Tobias, if you squint). I'm hopeful that the Celtics can find a way to upgrade at center, but even if they don't, they could match up better than you'd think against those teams with Kanter on the floor.