They're like Chelsea's Frank Lampard and John Terry, he says
The duo were unable to play together regularly last season owing to Ferdinand's injury worries, but are now manning the back line as the Red Devils have risen to the top of the Premier League standings.
"I think United’s league position has most to do with Rio and Vidic," May told the Daily Star.
"They have been playing together regularly and they’re the most important players at United.
"They were badly missed last season. I think Rio only played 13 league games.
"It’s like Chelsea being without John Terry and Frank Lampard. It has caused them real problems."
United are the only team in the top-flight not to have lost a single game this season.