Complex sentence: In 1986--87, he joined fellow league side A.C. Milan, being first-choice for three of his four years, and being the starter in the club's back-to-back European Cup conquests, in 1989 and 1990.
Split into simpler sentences: In 1986--87, he joined A.C. Milan, being first choice for three of his four years. Galli was the starting goalkeeper in the club's consecutive European Cup conquests, in 1989 and 1990.