Strictly speaking, yes Boston could go out and get Rubio with their non-Jays stuff. However, it probably comes down to either sending out Smart or Thompson plus one of the worthwhile young guys, and I don't think either trade really makes sense.
Personally, I don’t think it would take that much.
The math kind of requires it. To take back Rubio over the off-season Boston has to send out about $14.15 million. Thompson+Grant+Carsen doesn't quite get you there, and there isn't really a good reason for Minnesota to do that anyway. Thompson almost certainly pushes for a buyout right away, so they have to get something out of it.
Any concoction of transaction with Tristan Thompson included (even isolated by himself) works via RealGm.
The cap and trade rules can be complicated (I am no expert) but in this case, we are over the cap and we don't have a TPE that would fit Rubio. That means we can only take back 125% of what we send out (plus $100,000). So the "math" is that if we want to take Rubio at $17.8M, we need to send out at a minimum $14.14M. I believe someone else came up with a similar number.
I do not believe we can trade Thompson for Rubio straight up (and I wouldn't want to add that much salary but that is another point). We have an $11M TPE so we could get someone with that in a separate trade, package that $11M player with Thompson and get Rubio, which would effectively be Thompson for Rubio but we still end up adding about $8M while getting a player I don't really like for a player we don't really need.
Of all the parts of this deal, giving up Thompson is not the issue. If Rubio's salary was the same as Thompson, maybe but we would add $8M to do the deal plus lose our TPE. I see no reason to do this.