I'm getting the impression that the only real remedy for Porzingis' injury is surgery.
Rest alone is not going to put the tendon back in place.
Repair it immediately after the finals are over and spend the summer healing.
The question is, can he do more damage by playing on it now ?
A day or a week won't do much to repair it. They just have to decide if it's worth risking further damage if he plays, and if he does can he be effective or will he be hobbled. It's not the injury per se that's the concern as much as the risk that more damage to his foot could occur, with the worst case being a posterior tibialis dislocation. That's why he's not hobbling, he's shooting in practice, because it's not injury itself as much as what could be aggravated as a result of it which is the issue. The flexor retinaculum is a stabilizing tendon, it helps to hold the posterior tibialis (which is one of the most important tendons in the foot, which connects the calf muscle to the foot) in place. If it is torn or damaged it increases the risk of other tendons in the foot, including the posterior tibialis, to shift out of place. If you do then you've got a big problem. Those types of dislocations are very rate - but they do happen.
That's what the medical team has to make a determination on - what would be the % chance that the loads that KP would put on his foot would be enough to cause a more serious injury, including a dislocation? Can they stabilize it enough externally to reduce the risk to something that is acceptable?