"Northampton looked like a team that had something to play for – and we didn't," said McGhee after a barnstorming start from the Cobblers put them three goals in front.
"We were off it, looked uncertain in the first 20 minutes and got punished for it. Even though we were behind, I was disappointed we didn't have the confidence and belief to keep passing the ball."
Northampton never do things the easy way however and were given a scare by the visitors after the break.
McGhee added: "In the second half, we had more tempo, although all we could do was recover a bit of pride – and I think we did that. We made some changes, which were effective, and found a bit more energy."