Ryan said Willingham hurt his ankle a few days ago, but it flared up again during warmups before Monday's game against the Rays.
Willingham was held out for precautionary reasons, as Ryan said the injury is so minor that he didn't even know which ankle it was that was bothering Willingham.
"I don't know, because it's not very serious," Ryan said. "So I don't really care. He did everything [pregame], but unfortunately, he said he tweaked it just a little. So we don't want to mess with that. As usual, he said he could play today, but we didn't want him to play."
Ben Revere started in left field in Willingham's absence and batted seventh. Revere was originally slated to be the club's starter in left this season with Willingham in right, but the Twins opted to make Willingham their left fielder, as that's where he's played most of his career.
Revere is now likely to be the fourth outfielder, with players such as Trevor Plouffe, Ryan Doumit and Chris Parmelee competing for time in right. Plouffe and Doumit are locks to make the club, while Parmelee is gaining momentum after a strong showing this spring.
Ryan added that the Twins won't hesitate to give Revere playing time in right field despite his limitations with his throwing arm.
"It's not an ideal right-field arm, but it's not an ideal left-field arm, either," Ryan said. "But his arm strength is not going to hold him back from making this ballclub. He has other attributes that he provides for this club."