Manchester United striker Rashford and Manchester City fullback Walker are both targets for Milan this month.
Zola told La Gazzetta dello Sport: "Talking about Rashford is... a sore point for me because, ever since he was a kid, I've always considered him a strong player, someone capable of making the difference. Instead, he disappointed me a bit because he could have given much more: he alternated great seasons, like the one two years ago, with disappointing seasons like the last one. He was quite inconsistent.
"He is certainly fast and technically strong. In Italy he could have a significant performance, but it depends on him: for me he could have been the Premier League MVP for years and instead he never was.
"Talking about Walker is like discovering hot water: he is a physically top-level footballer and from this point of view he could make the difference in Serie A too. He would come from a tournament that is the most physically demanding in Europe and therefore he would be a defender capable of having a great impact."
On Milan moving for the England pair, Zola added: "This type of evaluation is up to the Rossoneri directors, who know Conceiçao's requests and the team's needs well, not to me. I will limit myself to saying that we are dealing with two top-level players, two players who in Italy could make their contribution to an important club like Milan. Indeed, who could make the difference."