Rosenberg said the injury is not life-threatening.
Investigators were reviewing footage to see if any security camera captured the attack, but Det. Larry Burcher said so far they've found "nothing of great value."
He asked anybody who may have filmed the beating with their cell phones or video cameras to come forward.
"We're very confident there were witnesses, it happened immediately following the game when everybody was coming out," Burcher said.
The Dodgers said they were cooperating with investigators.
The stadium has been plagued by Opening Day violence in the past.
In April 2009, a man stabbed his friend in the stadium parking lot after the home opener, in which the Dodgers beat the Giants. Arthur Alvarez was arrested and charged with assault with a deadly weapon. Alvarez, who contended that he was knocked to the ground and acted in self-defense, was later acquitted by a jury.
* St. Louis Cardinals outfielder Matt Holliday had an appendectomy and the team is unsure how long he will be out. General manager John Mozeliak said the surgery was not an emergency procedure.
* The Atlanta Braves acquired infielder Marcus Lemon from Texas for cash considerations.
In yesterday's games:
* At Miami, Josh Johnson took a no-hitter into the seventh inning, John Buck hit a grand slam in his Florida debut and the Marlins beat the New York Mets, 6-2, in the season opener for both teams.
Logan Morrison also homered and Gaby Sanchez added an RBI single for the Marlins, who played their 19th and final opener at Sun Life Stadium. The team moves into a new ballpark in 2012.
Willie Harris had a leadoff double in the seventh for New York's first hit off Johnson (1-0). Carlos Beltran lined an RBI double and Ike Davis knocked in another run with a groundout off Johnson.
* At Chicago, Neil Walker hit a grand slam, Andrew McCutchen also homered and Kevin Correia pitched into the seventh inning as the Pittsburgh Pirates opened the season under new manager Clint Hurdle with a 6-3 victory over the Cubs.
Chicago's loss, in front of a Wrigley Field crowd of 41,358 on a rainy day, spoiled Mike Quade's first game as the Cubs' full-time manager.
* At Denver, Matt Lindstrom's wild pitch with one out in the top of the 11th inning allowed Gerardo Parra to score from third base, sending the Arizona Diamondbacks to a wild, 7-6 win over the Colorado Rockies in their season opener.
Parra singled off Matt Reynolds to start the 11th, and Lindstrom came in and struck out a pinch-hitter before Willie Bloomquist singled to right, putting runners at the corners for Kelly Johnson, who was at the plate when Lindstrom threw the wild pitch.
Righthander Sam Demel (1-0), the fifth Diamondbacks pitcher, worked one scoreless inning for the win, and righty J.J. Putz got the last three outs for his first save.