Peterborough won thanks to goals from McLean, Boyd and and og from Steve Evans. The defeat keeps Wrexham in the bottom two.
Ferguson missed out on getting the managers post at Wrexham and took the oportunity at Llondon Rd when it came up.
"I was a bit disappointed not to get the Wrexham job, but I had no divine right to it and I was glad Brian (Carey) landed it because he's a good friend of mine," said Ferguson.
"The following week I got the Peterborough job, and I have to say that this club has got far more potential.
"But I really don't think Wrexham will go down. Brian is a winner, he always has been, and he will get the best out of the players."