The 32-year-old only made the switch from Aston Villa to the Riverside in the summer but has found first-team opportunities hard to come by since being replaced by Victor Valdes in the starting line-up.
Keen to be at a club where he is number one, reports in the United States have claimed Guzan is ready to end his stint in the Premier League and move back home.
Several MLS sides are thought to be interested in the shot stopper's services, including Chicago Fire, while expansion teams Atlanta United and Minnesota United, who are set to enter the league next year, are on the lookout for a goalkeeper.
Boro are unlikely to stand in Guzan's way if he is to push for a move with Valdes and Dimitrios Konstantopoulos available to boss Aitor Karanka.
Middlesbrough last week