1. Celtics view Evan Fournier as a long term piece.
Even though the Celtics only traded Jeff Teague and two 2nd rounders for Fournier, they made a bit of a commitment to the Frenchman since they used a good chunk of the TPE to bring him in. They also traded the one free agent this summer you'd want to keep in Theis, so they were also choosing Fournier to be the free agent you try and retain. They weren't going to pay both Fournier and Theis most likely especially, because of...
2. Tristan Thompson will pick up his $9.