The boss believes that the midfielder decided to make a return because he was missing the day-to-day life of being a footballer.
"It has been in the offing for a few weeks," Ferguson told ITV Sport. "Paul came to see me and said that he wanted to come back. He said he was missing it too much.
"There are no negatives. He is one of the greatest players in Manchester United's history."
Scholes, who is on the bench for today's Manchester derby, will wear the number 22 shirt.