Goals from Adam Eckersley, Richie Jones and Fangzhuo Dong gave United a 3-1 victory after Rene Howe briefly levelled for Peterborough in the second half.
But the return of Canadian-born Hargreaves, who came on for the second half, was a definite boost for Ferguson as he prepares to defend the Barclays Premier title they won last season.
Ferguson told MUTV: "It was good to see Owen back. He would have played the first half if we had not been delayed but I put him on in the second half so he could warm-up first."
Hargreaves said: "I am looking forward to playing 60 minutes in Scotland on Wednesday. I hope to be ready for the first game of the season against Reading."