Fabio Carvalho struck in injury-time for the Bees as they drew 2-2 at the Gtech.
"They obviously made a few changes, Brentford, to try and chase the game," Redknapp told Sky Sports as he began to break down Brentford's equaliser.
"He brings on Carvalho, he brings on (Kristoffer) Ajer as well. With (Michael) Kayode going off who is throwing the ball with such height and pace, I didn't even realise they had another long thrower in their midst with Schade.
"He launches one into the box, you can see you've got (Nathan) Collins and Ajer there, and what happens as it develops is this is the key battle here. Garnacho has got a job to do. He's got to mark Carvalho.
"That's your job, simple as that. Don't lose him, you're now one of the defensive players in here. If he gets in behind you, it's on you.
"Everybody in there is trying to win that first contact, it's not easy. Brentford have got some big guys. But Garnacho just falls asleep at the vital moment.
"The ball comes in from Schade, Garnacho's not concentrating, they get that contact and there he (Carvalho) is at the back post."