The Rossoneri have agreed a deal with Real Madrid for the 32-year-old, who has been relegated to third-choice at the Santiago Bernabeu, behind club captain Iker Casillas and new signing Keylor Navas.
Lopez has been left enraged by his demotion, having been coach Carlo Ancelotti's preferred No.1 in La Liga last season, and will seal a switch to San Siro on Monday.
Consequently, while the former Sevilla shot-stopper has been deemed surplus to requirements in the Spanish capital, Galliani believes that Milan have pulled off a major coup.
"He's the strongest goalkeeper in Europe," the Rossoneri CEO is quoted as saying by the Gazzetta dello Sport.
Lopez, who displaced Casillas as Madrid's first-choice goalkeeper under former Blancos boss Jose Mourinho, is set to sign a four-year contract with Milan.