After a tremendous campaign helping to offset Porziņģis’ various absences, Horford will be the one rotation Celtic entering free agency this summer. Replacing this season’s $9.5 million salary with a minimum contract next season would certainly help with Boston’s tax bill, too, but you can certainly understand why the Celtics do not relish that thought. Boston, mind you, has lost Horford in free agency before and more than one rival cap strategist out there has indicated that the 38-year-old could well attract offers starting at the projected $6 million taxpayer midlevel exception.