The Timberwolves announced the decision Monday. Free agency begins Tuesday, but contracts cannot be finalized until July 10.
The 6-foot-8 Hummel played in 53 games last season. The 2012 second-round draft pick averaged 3.4 points, 2.5 rebounds and 12.4 minutes per game.
The 6-foot-5 Jeffers played in two games in April after signing with the team for the final week.
Forward Dante Cunningham is the only other unrestricted free agent the Timberwolves have this summer. They have 12 players under contract, plus two draft picks.