Following Lazio's and Milan's relegation to Serie B due to their involvement in the 1980 match-fixing scandal, he was called upon by the Milanese squad and transferred to the side, becoming a first-team member alongside captain and defender Franco Baresi, as well as Filippo Galli, during a dark period in Milan's footballing history, under manager Nils Liedholm. He made his Milan debut on 24 August 1980, in Serie B, in a 1–0 home win over Catania. Milan won the Serie B title that season to return to Serie A, with Tassotti emerging as one of the club's best performers. Despite winning the Mitropa Cup the following season, Milan were unable to remain in Serie A, finishing third last, and were relegated once again to Serie B. Milan were able to win Serie B again the following season to return to Serie A. Although this was a fairly bleak period in the club's history, as they failed to dominate in the league or capture a trophy, the squad did reach the Coppa Italia final during the 1984–85 season, and were able to regularly qualify for the European competitions, frequently finishing in the top half of the table.