Saturday's League One match was settled by Charlie Daniels' penalty five minutes after half-time to leave North End with a solitary win in their last 10 outings.
Westley said: "Honestly I'm just glad to get to the end of the season. This isn't a today thing. This is something that's been endemic over the majority of the season. Nine defeats in 11 in that period before I came in - you know there's some cancerous problems in there when you inherit something that's got that sort of performance.
"It's been tough driving the thing along and trying to eke out some points to make sure we weren't suffering a back-to-back relegation.
"You've got to have people that want to do it your way. If you've got all of these very basic problems, then you're working with the wrong group, and that's very definitely in my opinion the issue here.
"It's not a secret that I want to change the squad around. We've got too many players in my opinion who aren't up to the job. We need a major overhaul, there's no question. We can't produce performances like we've done over the course of the season."
