Sir Alex Ferguson's final signing as United boss was expected to make a big impact at Old Trafford this season.
Instead, the £15m arrival from Crystal Palace made just four appearances including the Community Shield, and has now moved to Cardiff on loan until the end of the campaign.
Zaha made an impressive debut for Cardiff in their win over Norwich last weekend, which has led to some critics wondering why the 21-year-old has not featured more for United.
But Moyes insists competition on the flanks has limited Zaha's outings with Adnan Januzaj, Shinji Kagawa, Nani, Antonio Valencia and Ashley Young all battling for places in the first half of the season.
"At times you have to believe what the manager is seeing as well," Moyes said. "There is big competition for those wide areas.
"When I came in, I had Antonio Valencia, Nani and Ashley Young. Then we had the emergence of Adnan. Shinji can play off one of the sides.
"I have seen Wilfried training every day as well. I am privy to that."
"We might have got him on loan even earlier but for the (hamstring) injury to Nani. I just wanted to see how that was.
"It is good for him. It is good for his development. Everybody will have a chance to see how he plays.
"The idea is he shows what he can do and comes back a better player for Manchester United."