McGuiness made the revelation when discussing succeeding Sir Matt Busby as United manager.
"There were too many grey areas between us," said McGuiness. "I was never sure what was his responsibility and what was mine but they were not the reasons I failed.
"The main reason was a lack of transfers. I signed one player, Ian Ure. I wanted to bring in Mick Mills from Ipswich and Malcolm Macdonald but I was told that the fees were too big."
"The lesson of Manchester United is the longer you manage, the easier it becomes."