With Issac missing all of the 21-22 season, his contract now has decreased to 16m in 22-23, and only guaranteed 7.4m the next season.
Likely, Gallinari would be waived this off season for 5m. In this trade, they take a chance on Issac, and if he doesn't work out, waive him for only 2m more than what they would have waived Gallinari for the year prior.
For ORL, they get a first out of someone who, after this draft, will most likely be coming off the bench anyways... If healthy.