Otamendi should have seen red in the first 2 minutes then. When he brought down Salah in a reckless and dangerous fashion but wasn't.By the book.
Fifa's laws of the game for the 2017-18 season contains the following entry...
"Serious foul play - a tackle or challenge that endangers the safety of an opponent or uses excessive force or brutality must be sanctioned as serious foul play."
So the refs don't follow their own laws.