Lukaku recorded 42 goals in 96 appearances for United, but did start to slip out of favour when Ole Gunnar Solskjaer inherited the managerial reins from Jose Mourinho.
Cordoba is delighted to see a prolific presence on the books of his former club, but has questioned why United were prepared to part with a player that clearly still had plenty to offer.
The ex-Inter defender told the Daily Mail: “Lukaku is an extraordinary player.
“Manchester United did not understand the player's potential and his last season was bad for him. In my opinion Lukaku is proving everyone to be a champion.
“He is determined and the Premier League has improved him a lot. I met him: he is a positive leader in the changing rooms.
“It's a shame that Manchester United sold him, but I'm happy that Inter bought him. Antonio Conte has been fundamental to his form at Inter.”
Lukaku has found the target on 23 occasions for Inter, through just 35 appearances.
He has also been joined in Milan by a number of familiar faces from his time in the Premier League.
Cordoba expects all of those deals to benefit Inter, saying: “Eriksen and Young are excellent players and will give determination and strength to Inter. I am happy that they chose Inter.
“Eriksen comes from Tottenham, a wonderful team, and Young was a symbol of Manchester United. Inter made two big purchases.”
Conte’s side are currently waiting to discover whether Serie A competition can resume amid the coronavirus pandemic, with Inter entering lockdown sat third in the table – nine points adrift of leaders Juventus with 13 games left to take in.