Ahead of the 2-2 draw, which came after Wayne Rooney and Javier Hernandez had United two goals inside 15 minutes before Eden Hazard and Ramires levelled for the Blues, Ferguson failed to shake Benitez's hand before kick-off.
"I was waiting at the beginning. It's up to him, I was in the way waiting," the former Liverpool boss said.
Asked why that might have been the case, Benitez added: "Ask him - if you are brave enough ask him. It was his decision. I was ready and waiting. I have some education because I know a lot of people are watching so I know what I have to do."