The slugger had the shot after Saturday's game, with the Twins scheduled for an off day Monday. Willingham and manager Ron Gardenhire said Sunday they expected him to return Tuesday against the Chicago White Sox.
Willingham leads the team with 10 home runs. He played in 59 of the Twins' first 65 games despite the sore knee. A .258 career hitter, Willingham has slumped to a .211 average this season.
He says: "I wish I could blame my hitting on it, but I can't."
Willingham is just 3 for 27 with no homers or RBIs in his last seven games. Saturday's shot was Willingham's first this season.