United - who finished seventh last term - have splashed the cash in the transfer window in a desperate bid to return to former glories.
They parted with over £150 million to bring the likes of Angel Di Maria, Luke Shaw, Ander Herrera, Marcos Rojo, Daley Blind and Radamel Falcao to the club but have won just one of their first six matches in all competitions.
And despite holding a 3-1 lead at Leicester with just 28 minutes to play, Louis van Gaal's side crashed to a 5-3 defeat at the King Power Stadium.
A thorough review was held afterwards and striker Van Persie said nothing was spared in the lengthy discussions.
"It shouldn't be possible [to lose like we did]," Van Persie told Fox Sports News.
"But it still happened. We have to deal with it.
"We had a proper look at it next day. Sometimes it was a bit confronting and it was a long meeting. One hour [or] something.
"We went from there, started again this week [and have put] a couple of really good sessions in. Everyone feels ready to face West Ham."
Sam Allardyce's West Ham are the visitors at Old Trafford on Saturday and will be given confidence by the fact their opponents have lost six Premier League games at home in the last year.
The under-fire hosts have kept just two clean sheets in their last eight league matches - a run West Ham will be looking to continue.
But Van Persie said the club's review pointed blame at the whole team for their defensive errors - not just the back three or four, depending on what system van Gaal uses.
"When you look at the defensive mistakes, it starts somewhere else," he added.
"That's where we looked at as well. It's not just the one mistake which happens. There's a mistake before the mistake and before the mistake, maybe positional wise. That's where we looked at.
"Everyone knows where we have to improve."
