Manager Kirk Gibson said Trumbo will get a second opinion from Dr. Michael Lee in Arizona before a timetable for his return is determined. An MRI exam Wednesday revealed the injury after an X-ray showed no structural damage.
San Diego Padres starter Josh Johnson is scheduled to undergo elbow ligament-replacement surgery for the second time and will sit out the entire season.
The right-hander, signed in November to an $8 million, one-year contract, was put on the disabled list before the season began. He strained his elbow during spring training and the Padres initially thought he would be out about one month.
The Oakland Athletics said they are stopping negotiations to extend their lease at the Coliseum.
A's President Michael Crowley issued the statement after the Oakland-Alameda County Coliseum Authority said the team owes more than $5 million in back rent.
Crowley said the team owes no money and said rent payments were deducted in the past for items allowed under the terms of the lease.
The A's current lease at the Coliseum expires after the 2015 season, and the team rejected a 10-year extension offer on Tuesday, saying the deal "did not address all of our issues."