Here's the package they could offer without Brown or Tatum.
Marcus Smart (sign and trade for 12.5 million a year, worth 6.25 in salary as part of this trade)
Terry Rozier
Marcus Morris
Abdul Nader (non-guaranteed)
19 Sac Pick
Given that the Cs have bigger assets (Brown and Tatum), I think the Spurs would want more. That said, if the Celtics didn't have bigger assets, that package is not bad.
That is a two-way veteran role player that would fit the Spurs system really well (Morris scores in the places on the court that the Spurs attack), a All-Defensive talent, a young guard with the potential to be a long-term starter in the NBA, and the 19 Sac pick that could be top 5.
I think you could argue that is better assets than what the Spurs would want from the Sixers (they won't want Fultz because the Spurs were one of the first teams that considered personality as part of their scouting process). Covington roughly equals Marcus Smart (depending on your personal preference), Saric roughly equals Rozier (depending on your personal preference), and the 2018 Sac pick is better than any draft picks the Sixers can offer.
Eastern Teams like the Heat, Cavs, Bucks, Raptors, Pacers, and Wizards can all make an offer with comparable or better rotation players, but worse draft assets.
The Cs are probably in the mix, but the above trade means they have an unbalanced roster and it pushes Brown or Hayward to come off the bench. They'd need to still sign another big (Baynes, Monroe, or O'Quinn) and a couple point guards (Larkin, Evans, etc.).
If Ainge pulls the trigger, I could get on board with it, but I'm completely fine with the current roster next season.