The Giants restricted Morris to just 26 yards in their victory at FedEx Field, while Griffin was sacked five times during the contest.
"We thought going into the game the cowbell was Alfred Morris," Beason told the Star-Ledger.
"You look at when he has a big game vs. when he doesn't have a big game, even when RG3 has a big game, they don't do too well. He's the guy that makes them go."
The Giants are 5-7 in their season after week 13.