The 21-year-old joined the Red Devils in January 2013, but has made only four appearances for the club.
He has since moved back on loan to the Eagles, and Warnock believes that the winger has found it difficult to adapt to life at Old Trafford.
"Man United was probably a bit too big really," Sky Sports News quotes Warnock as saying. "They haven't got time to educate players as much as a club like Palace have.
"We bring through players like Wilfried all the time and they need educating.
"To take him as quickly as they did and with the money involved, it was very difficult I think."
Zaha had a spell on loan at Crystal Palace last season, but failed to find the net for the Bluebirds.