Though the Magpies played admirably in the first half at Fratton Park and were unlucky to concede just before the break, they were run ragged in the second half as Kyle Bennett, Christian Burgess and Marc McNulty weighed in with simple finishes.
The former Swindon boss said: "A lot of people told me before I came here that this team conceded a lot of goals and couldn't defend properly.
"I think you've just seen (on Friday) that the softest of goals unlocked us and changes the complexion of the game.
"All four goals aren't from brilliant play from Portsmouth but ridiculous defensive errors. The players need to take some responsibility; they've had three managers.
"The first goal, their lad has picked up the ball 15 yards into our half, run down the touchline, cut in and rolled the ball into the net - that wouldn't happen tomorrow on the park.
"It's like park football - it's not just defenders, the players need to take responsibility. I can't take any satisfaction from that other than 11 men stayed on the pitch."
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