Fellaini had been booked moments earlier before he and the Argentine striker came together, resulting in a straight red from referee Martin Atkinson.
"What is he doing? What is he doing? What is he doing?" Neville said on Sky Sports.
"Before that he's just taken the yellow card for stopping the counter attack.
"I think he's furious because what he's done is that he's given the foul away and there is an incident we haven't seen. He's snapped Fellaini, he needs to get off the pitch.
"Aguero's played him there like a fool. What is he doing? He's played him like a fool.
"He's now left his team with 10 minutes to go, maybe more, with a lot of work to do."