Adkins is ready to dip into the transfer market after the Sky Bet Championship side were embarrassed by the League One high-flyers in a London Road rout.
The Royals manager had made wholesale changes to his starting line-up, and the decision quickly appeared to have backfired as hat-trick hero Lee Tomlin led an onslaught that earned Darren Ferguson's men a sixth successive victory at the start of this season.
Adkins said: "Peterborough deserve a lot of credit for their performance, but that produced by my players was not acceptable.
"We came here with the desire of progressing in the cup but have gone away with a very heavy defeat despite having seven or eight internationals on the pitch.
"This game is about scoring at one end and keeping the ball out at the other. We did neither of those things and I can only applaud our travelling supporters because it was not a nice night for them.
"It was a rude awakening for a lot of players and they know there will be consequences.
"There are a few days left in the transfer window and I will be speaking to the owner on Wednesday."
Posh boss Ferguson was left to savour one of the best performances of his managerial reign.
He said: "It was an exceptional display from start to finish and all the way through the team.
"Our counter-attacking football was fantastic and beating a club of Reading's calibre so heavily has to be up there with the very best performances we've had at this club.
"We have to enjoy it as it was one of those rare occasions where every one of our players put in an eight or a nine-out-of-10 performance."