The former Roma boss has claimed that the English giant's manager contacted him ahead of a potential move for the midfielder.
The Red Devils turned their attention to De Rossi after missing out on Thiago Alcantara and Cesc Fabregas, but the transfer never materialized as Roma refused to sell the 30-year-old in the closing stages of the summer transfer window.
De Rossi made his official Roma debut under Capello in 2001-02 and the current Russia coach holds his former charge in high esteem.
"David Moyes asked me for my opinion about De Rossi a few months ago and my opinion could only be positive," Capello told Fox Sports.
"If I were De Rossi, I would have gone to Manchester United and join a team where they have a lot of ambition."
Manchester United eventually strengthened the midfield with Marouane Fellaini from Everton on deadline day of the transfer window.