I am absolutely mind blown that there are people here who would jump at the idea of trading Tatum for Siakam.
Wow.
Siakam's Per-36 numbers over his first two seasons (at age 22/23):
11.6 points, 7.8 rebounds, 2.5 assists, 1.25 steals, 1.23 blocks, 1.4 turnovers, 3.9 fouls, 50% FG, 22% 3P%, 64% FT
13.0 PER
Tatum's Per-36 numbers over his first two seasons (at age 19/20):
17.33 points, 6.47 rebounds, 2.18 assists, 1.35 steals, 0.85 blocks, 1.74 turnovers, 2.49 fouls, 46% FG, 40% 3P%, 84% FT
15.2 PER
I've intentionally used Per-36 numbers to compensate for the fact that Siakam played far less minutes over his first two seasons, to help make the comparison more fair. But even on a Per-36 basis Jayson Tatum clearly produced at a higher level then Siakam did over their first two seasons, despite being THREE years younger.
Even last season, when Siakam had his major breakout year, their numbers still weren't that far apart
Tatum (21 years old, 2nd NBA season, still learning): 15.7 PPG, 6 RPG, 2.1 APG, 1.1 SPG, 0.7 BPG, 1.5 TPG, 45% FG, 37% 3PT, 86% FT
Siakam (24 years old, 3rd NBA season, breakout year): 16.9 PPG, 6.9 RPG, 3.1 APG, 0.9 SPG, 0.7 BPG, 1.9 TPG, 55% FG, 37% 3PT, 79% FT
Now obviously nothing is ever guaranteed in the NBA and you never really know how things will turn out, but from what I can see it seems blatantly obvious that Tatum has a far higher ceiling then Siakam. Siakam's breakout-year stats weren't THAT much better then Tatum's as a sophomore, and we haven't even SEEN Tatum's breakout year yet. It could well be this year.
Trading Tatum for Siakam would be TITANIC mistake in my opinion, and one that I very strongly doubt Danny Ainge would even consider doing. Siakam might be the better player right now and might win us more games this season (might: even that depends on how big a leap Tatum makes), but Tatum has a far higher ceiling and is a much better looking prospect and long term investment.