Team doctor Stephen Lemos at the Detroit Medical Center examined Willis on Monday afternoon after the left-hander had complained of reoccurring pain in the knee, which he hyperextended in early April. Willis received a cortisone injection to reduce inflammation.
He will return to Lakeland later this week to resume his program with the organization's pitching instructor, John Matlack.
With his permission, the Tigers sent Willis, the 2005 National League Cy Young runner-up, to Lakeland four weeks ago after a disastrous start against the Indians. He surrendered eight runs on two home runs and five walks in just 1 1/3 innings.
At Lakeland, he made one outing June 29, in which he allowed one run on one hit and three walks over two innings. He hasn't pitched in a game since.
In five outings this season with Detroit, Willis walked 21 batters over 11 1/3 innings, allowing 13 runs on seven hits with five strikeouts and recording a 10.13 ERA.
The Tigers acquired Willis in a blockbuster December trade that included Miguel Cabrera. They signed Willis to a three-year, $29 million contract soon after.
Scott McNeish is an associate reporter for MLB.com This story was not subject to the approval of Major League Baseball or its clubs.