The scene around Butler after the game supported those concerns.
Butler suffered a broken leg, the Seahawks said. The team will presumably have more details Monday.
Multiple doctors from both teams and various medical personnel surrounded Butler after the game in a side room adjacent to the 49ers' locker room at Candlestick Park. They spilled out of the room into the hallway. Butler was lying down inside the room, awaiting transport to a hospital.
Before heading from San Francisco's locker room to a postgame interview room, 49ers coach Mike Singletary worked his way through the doctors so he could share his concerns with Butler in person. Singletary did not stay long, but the fact that he felt compelled to visit Butler in person spoke to the seriousness of the injury.
"I was just out there and I know what I saw," Seattle receiver Ruvell Martin said. "You can see something sticking out of his leg, so it didn't look so good. ... He just looked at me and goes, 'My leg.' All I could do was just give him my hand and say a prayer for him and try to comfort him as much as I could."
Seahawks coach Pete Carroll said Butler was "unbelievably courageous and tough about it on the field" immediately following the injury.
Butler had just caught a 2-yard scoring pass with 1:55 remaining in the Seahawks' 40-21 defeat when a defender's leg collided with Butler's leg.