A big fat F, with F standing for FIRE instead of FAIL.
Fournier is the ultimate regular season fraud. I've went into his qualitative shortcomings against strong playoff defences even as a 3rd or 4th option in the Fournier thread, so here is some quantitative analysis of his play against playoff defences:
Per game averages in his last two playoff series (10 games)
12.6 points, 3.6 rebounds, 2.3 assists on 34.9/29.0/72.0 shooting, 46.0% TS.
The sample size isn't perfect, but I've watched most games in both series and Fournier doesn't look any different physically - the low hanging fruit that he grabs in the regular season simply got taken away by elite defences in the playoffs with just increased intensity from his man. And it wasn't like the Bucks or Raptors keyed in on him with their best defenders - Fournier was relegated to being the 3rd or 4th option in those two playoff series, so the Raptors rotated whichever of Green/Kawhi/Lowry/Siakam who was free on him while the Bucks used defensive legend Wesley Matthews to shut him down.
Now Fournier did pretty well against weaker defenders (eg. Norman Powell), but the best teams in the league mostly have strong defences with a stable of good perimeter defenders. The 76ers have Danny Green, Ben Simmons, Tobias Harris and Matisse Thybulle, the Bucks have DiVincenzo, Middleton, Holiday and Giannis, the Lakers have Caruso, Matthews (hello darkness my old friend to Fournier), LeBron and Davis, the Clippers have Kawhi, George, Batum and Beverley and the Suns have Paul, Bridges, Crowder and Johnson.
Obviously even a Fournier who struggles against strong playoff defences is a massive upgrade over our bench wings, but this is far from the 19 PPG guy people think we're getting, and I'm not sure if it's good asset management to spend 2 2nds and a big chunk of our TPE for him when the Bucks got PJ Tucker for a 2nd and salary filler (obviously Tucker is much older than Fournier, but I see them both on opposite ends of the offence/defence spectrum as 'good role players').
As for moving Theis for a considerably worse prospect and a useless centre, my thoughts are the same as what most echoed when the trade was made: we traded one of our best bigs for rubbish and little to no tax savings with zero assets in return.