Juve sold the Spain defender to Bournemouth a year ago, with Huijsen now a Real Madrid player.
Capello told La Gazzetta dello Sport: "I'm surprised that Juve didn't understand the obvious potential of the boy. Huijsen can't have suddenly become good.
"If at 20 he plays for Real Madrid, all the more so he's a Juve player."
Capello admits he's a fan of Juve midfielder Kenan Yildiz, adding: "Yildiz? He has quality and the humility of a champion: he's a talent on par with Real Madrid. Juventus are right to hold on to him: this could be his game."
Juve will meet Real Madrid tonight in the Club World Cup round of 16.