Aberdeen winger Mackay-Steven got his head to a Shay Logan cross, which he managed to divert goalbound.
As he did so, Boyata came in with a late attempt at a header, which saw the two players clash heads and Mackay-Steven knocked unconscious.
Though he regained consciousness on the field, he was removed from the action on a stretcher. Boyata stayed on after receiving lengthy treatment to a cut on his head and provided an assist for the opening goal, scored by Celtic team-mate Ryan Christie.
Some Aberdeen fans thought the Belgian defender should have been penalised. Twitter user @funkybeatts1903 asked: “Why is that not a penalty to Aberdeen? GMS wins that ball. Boyata takes him clean out with his head. Why is that not a pen?
Celtic fans were also less than impressed with some booing of Boyata from the Aberdeen support at Hampden. @weblivz said: “How Aberdeen fans can boo Boyata for that clash is beyond me. Horrific accident for the two of them.”