Managers tend to think in “group think.” Substitutions must be made at the optimal times – 60th minute, 70/75 minutes, and depending on the situation in the 80th or later. Rarely does a manager make a first half substitution except for injury, even if a player is rubbish. In this case Mitrovic was rubbish and in danger of seeing red. Yet Benitez gambled by keeping him on and holding out until halftime, when he could substitute him, if not later.
Read more here about the match.