The Switzerland defender is on a season-long loan at Inter from Manchester City, with the deal carrying a €15m permanent option.
For Capello, Inter management should have seen enough to convince them to trigger the clause.
He told La Gazzetta dello Sport: "He has shown great value, he has experience and quality. In some ways, he's a bit like Inter's Modric, although he hasn't yet reached his level.
"Would I buy him again? With a 30-year-old like him, absolutely, in a league like the Italian one, he can easily get another three years. In that case, Inter should ask themselves: what better could I find?"
On Luka Modric's impact at AC Milan this season, Capello was effusive: "He has superior intelligence, he senses the game in advance and therefore manages to get there early.
"Italian football is so slow that by the time he gets the ball, he's already read the book and closed it."