Zaha starred on debut for Cardiff in victory over Norwich City on Saturday.
"David Moyes has seen Zaha every day in training but we don't know how the lad operates on the training ground. We don't see everything that the manager sees," Thomas told the Manchester Evening News.
"We can only judge by what we see in a match and yes it was a decent start at Cardiff. But it is a totally different thing playing for Cardiff than it is playing for Manchester United.
"The intensity at Cardiff won't be anywhere near as tough as it is at Old Trafford. He won't have had two players on him. Despite how crucial that match was Norwich won't have been as fired up and intense as if they were playing United.
"It is just one game and I don't think anyone should be saying that David Moyes has called it wrong about Wilfried just yet. It is a tough one for Moyes because with United not winning people are now saying Wilfried Zaha was the answer! It is an obvious reaction after the weekend.
"Let's see how he continues down there. He's got to keep performing and build on that before you can say he will do well for United in the future."
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