According to some sources this morning there is a 90% chance that Nathan is done for the season.

Minnesota Twins All-Star closer Joe Nathan has a torn ligament in his throwing elbow.
General Manager Bill Smith says the plan is to rest Nathan for two weeks, eliminate the soreness and build strength in his elbow before making any decisions about surgery.
Nathan was 2-2 with a 2.10 ERA, and had a career-high 47 saves last year.
Nathan may get a second opinion from Dr. James Andrews, the surgeon who operated on his elbow last October to remove bone chips.
Nathan hurt his ulnar collateral ligament. Injuries to that ligament have led to season-ending Tommy John surgery in other pitchers.
Nathan said he'll try to come to a decision quickly.
"I'm going to do whatever I can right now to give myself the best chance to go out there and help this team out," Nathan said, according to the Minneapolis Star-Tribune. "But at the same time, I'm not going let this go on too long because at the same time, I know these guys have a decision to make, and they've gotta prepare for the season, too, so I will make a decision quicker rather than later and give them ample time to do what they have to do."
Nathan flew back to Minneapolis Sunday after feeling soreness in the elbow in Saturday's game against Boston.