Wells had been on the disabled list since May 26 after being hit on the right knee with a line drive by Tampa Bay's Travis Lee. He entered Monday's game with an 0-1 record and 8.64 ERA in two starts.
Nixon was hurt when he swung at a pitch during Sunday night's 10-4 loss to the Los Angeles Angels and left the game immediately. He's batting .297 with seven homers and 47 RBI.
General manager Theo Epstein estimated Nixon would miss three weeks.
The severity of the injury "is moderate," manager Terry Francona said. "It's not insignificant. It's not horrible. It's right in the middle."
Wily Mo Pena started in right field Monday and is expected to be Nixon's primary replacement. In the home opener this season, a ball went off Pena's glove for a home run in right field against Toronto.
"His first day here didn't go as planned," Francona said. "In Boston, that can be the end of the world. He'll be fine."
The Red Sox obtained Corey on Sunday from Texas for pitcher Luis Mendoza, who was 1-5 with a 6.38 ERA in nine starts at Double-A Portland. Corey made 16 appearances for the Rangers, going 1-1 with a 2.60 ERA.
Van Buren was 1-0 with a 10.80 ERA in nine relief appearances with Boston.