Wolves manager McCarthy has come under fire for leaving all 10 outfield players from the weekend win at Tottenham out of their clash with Manchester United.
Arsenal boss Arsene Wenger was so incensed he claimed United - who recorded a 3-0 win - were effectively embroiled in a 37-game campaign compared to 38 for all their rivals.
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To see the entire outfield team changed was a surprise to Ferguson, but the basis of the decision was not.
Having assessed the stats of Wolves' players at White Hart Lane, the Scot concluded at least six of their team would not be asked to produce a repeat showing at Old Trafford.
'Mick McCarthy did the right thing for Wolves,' said Ferguson.
'We looked at their performance against Tottenham and the amount of running they had to do. I said five or six of those players could not play.
'In addition to the actual match, they had been down to London, then they had to travel to Manchester and they also play Burnley at the weekend.
'We didn't think they would play their full side. We worked through the ones who did all the running and worked out who we thought it would be.
'We didn't expect 10 but we weren't far wrong.'
Since Tuesday night's encounter, a number of former professionals have expressed their disgust.
A number point to the great Liverpool sides of the 1970s, who often featured in 60 games a season without the need for a rest.
Yet Ferguson believes there is a fundamental difference in the game nowadays, both in terms of speed and physicality and the technology that allows clubs to monitor their players for evidence of a drop in standards.
And the results, according to United's long-serving manager, are what makes Sir Bobby Charlton's appearance record staggering.
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'Bobby Charlton played 759 games for Manchester United, without substitutions,' he said.
'Ryan Giggs has reached 821 games, but, both coming off and going on, he has been involved in hundreds of substitutions. That tells you how the game has changed.
'Up until 1965 there were no substitutions. After that it was only one, then two.
'To play 759 games in that era is amazing. It could never be done again.
'Ryan has benefited from being a substitute 112 times. That is an indication of the speed of the game and the competitive element.
'There are also more games now. Okay, there are benefits. The modern technology, sports science and nutrition have all improved a players' ability to last longer and deal with injuries better.
'Nonetheless there are a lot of games played at a high tempo.'
And, rather than effectively providing his team with a walkover on Tuesday, Ferguson feels the changes worked to Wolves' advantage.
'The Wolves side that played on Tuesday did exactly what a lot of sides have been doing to us recently,' he said.
'They never gave us a minute on the ball. They worked their socks off.
'To my mind, there was no discernable difference in the Wolves team that could have played to the one that did.'
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