"We're confident of our position on this and have cited the example of three or four other teams doing something similar.
"If we'd fielded five youngsters then I could understand the risk of us being in bother. But I think with the exception of George Fried, it was the same 20-man squad that travelled to Tottenham."
Wolves beat Tottenham 1-0 days before their visit to Old Trafford, with McCarthy also citing the nature of the victory over Spurs as another reason to rest key players.
That victory over Spurs was the second successive top-flight win for Wolves - the first time the club have achieved that in 26 years.
But McCarthy responded to the victories by keeping faith only with keeper Marcus Hahnemann against United, explaining afterwards that the physical exertions of the players at White Hart Lane ensured they needed a rest.
The second string went on to lose comfortably, leaving many Wolves fans bemused and frustrated.
McCarthy's selection against United was branded "grossly disrespectful" by Wolves supporter chief Arthur Williams.
And several of the 3,000 Wolves supporters that travelled to Manchester also asked for their money back.
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