An offside call is not a subjective call. A player is, or he isn't offside. And if he is, the goal is disallowed.
Now, the question is: When is a player considered offside?
FIFA has an offside rule, which is regularly updated and refs, coaches, players are informed about the updates.
Here it is:
https://www.fifa.com/mm/document/afdeveloping/refereeing/5. law 11_554.pdf
You can read it all, but I will cut it down to the important parts regarding Saponara's disallowed goal:
- A player is in offside position if: Nearer to his opponents’ goal line than both the ball and the second last opponent
at the moment the ball TOUCHES or is played by one of his teammates (page 3)
- Gaining an advantage by being in offside position means: Playing a ball
that rebounds to him off an opponent, having previously been in an offside position (page 11)
So, Saponara was offside at the moment the ball touched his teammate AND at the moment the ball rebounded to him off an opponent (Alex Sandro).