The marauding striker re-injured his ankle in last week's Champions League quarter-final return against Bayern Munich after having hurt it in the first leg.
The English champions were knocked out on away goals after a 4-4 aggregate score.
The problem is not considered serious and the player, who has scored 34 goals for United this season and is vital to England's World Cup hopes in South Africa, is expected back soon.
"I understand Ferguson perfectly," Capello told Thursday's La Gazzetta dello Sport. "He knows how important Rooney is psychologically for the whole team.
"I don't think he took a risk. If he fielded him after the tests, obviously it wasn't serious."
Capello, who has a reputation as being a strict disciplinarian, also insisted England's World Cup camp will be alcohol free and a like a spell of "detox" for the players.
"It's nothing new. All England's training camps have been dry with me," said the Italian, who did not rule out a return to club management when his England deal expires in 2012.
"The ban is valid for me and the (backroom) staff too. It will be like going to a health centre. We'll detoxify in the hotel."
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