This is getting deep into the weeds of the salary cap, but basically we are constrained by the fact that we are only $842,292 over it and we need to fill two roster spots:

For people in the buyout market, if we were to offer them a vet min contract, the exact number of that depends on their years of service. Here are the tiers, and the amount prorated that we would pay if we signed someone from the buyout market in the next week or so.

You can see how tight the numbers are - if we sign anyone other than a rookie min as our 13th, we would incur at least $789m, which would put us only $53.2m under the tax line. And we need to sign the 14th within 14 consecutive days of Feb 5 (make that Feb 18 as our "deadline" to have 14 players). Pro-rating a rookie vet min won't get us to $53m so we would go over the tax.
What they will most likely do is convert one of the rookie 2 ways in Shulga to a standard min at $490m, like Amari, which would bring us to $352m over the cap. In fact signing Harper Jr to a vet min on Feb 18 would probably cost approx. $395m, which would put us over the cap, so they might have to do some funky stuff like continually signing someone to a 10 day contract until the pro-ration for the permanent 14th player gets low enough for us to sign him. I didn't want to go through all the calculation as it's a) extremely tedious; b) extremely boring for everyone here and c) extremely depressing when you realize how limited we are if we want to make a big splash in the buyout market.
TLDR: Long story short - don't expect to see anyone other than a rookie signed to one of those roster spots if we want to stay under the tax. I'm guessing the players who will be signed to them are already in our team - Shulga (now) and Harper (later, after some funky stuff with 10 day contracts, etc).If one of the cap experts like Celtics2021 or Jambr or Keevsnick can point out a flaw in my math I will be very happy to be proven wrong
