United faced Porto on Tuesday, just 48 hours prior to the Champions League tie.
Fergie had the chance to move their game with Aston Villa from last Sunday to last Saturday, due to the international break.
He said in The Sun: “We showed tiredness because of the gamble I took playing Sunday which was not the best preparation.
“But we faced a situation and we took the best option hoping a European occasion would bring the adrenalin up.”
Ferguson added in The Guardian: “I was tempted to get Villa moved to the Saturday.
“However, we had so many players on international duty in places like Bolivia, Rio de Janeiro and Korea, involving some very long and tiring journeys, I thought it best to go with Sunday.”
