They can afford one player in the buyout market, although doing so could put the Celtics into the tax if they win it all. I think the Celtics would accept that trade off, so ultimately I would be surprised to see Kornet on the roster in a couple weeks.
While the Celtics writers are being a little vague, the fact is that merely trading Green and Teague got the Celtics sufficiently below the tax line. Trading Theis for Wagner and Kornet saved enough additional salary to afford one rest of season free agent. So unless the Celtics thought either Wagner (possible but unlikely) or Kornet (no chance) were superior to Theis, that move was made to afford someone in the buyout market, and it would be surprising for them to not go after someone.