The Tigers optioned Willis to Lakeland of the Florida State League -- and home to the Tigers' spring training camp -- a day after he matched a career high by giving up eight earned runs on five walks and three hits in 1 1/3 innings against Cleveland.
Willis (0-1) has walked 21 batters in 11 1/3 innings. The 8-2 loss to the Indians came in his second start since going on the disabled list with a hyperextended right knee.
"Our objective is to get Dontrelle back to being Dontrelle," Tigers president, CEO and general manager Dave Dombrowski said. "We don't want to put a time frame on this. We want to get it right. The arm strength is still there, on occasion. The breaking ball is still there. But we want to get him back to being comfortable throwing strikes."
Detroit signed Willis to a $29 million, three-year deal after getting him in a trade last winter with the Florida Marlins.
"We knew his walks were up last year, but not anywhere near the extent of this," Dombrowski said. "We studied it very thoroughly. We knew he hadn't pitched as well the last couple of years but we didn't see this command situation."
Dombrowski also said no decision will be made immediately about who will replace Willis in the starting rotation.
"We'll tackle that in a few days," Dombrowski said.
The Tigers also recently placed right-handed starter Jeremy Bonderman on the disabled list with a blood clot in his shoulder.