The 36-year-old departed Milan in the summer to join MLS outfit Montreal Impact, bringing a 10-year stay at the San Siro to an end.
It had been suggested that the veteran could return to Italy in the MLS off-season, but Galliaini has quelled such claims.
"All the veteran players who left Milan are missing me because they are absolutely big champions who made Milan history and won everything," Galliani told Sky Italia.
"But now Milan's project won't allow for the return of a player who is close to the age of 37. Nesta is a formidable player but he was born in 1976 and so I think it could be difficult if he came back.
"I say this is year one of the new Milan. A sensational cycle has ended and we must able to allow players like De Sciglio, Bojan and El Shaarawy to grow. We play with a young side and there is a need for patience."
Milan currently sit 10th in Serie A.