Dareus was absent from practice as the Bills prepared for Sunday's Week 2 home matchup against the Kansas City Chiefs, The Buffalo News reported.
The player's brother, Simeon Gilmore, 19, was among three young men found slain Sunday in a Birmingham, Ala., house.
A 20-year-man who lives in the neighborhood has been charged with counts of capital murder and burglary in connection with the case, The Birmingham News reported.
Gailey suggested Dareus would likely be back in time for Sunday's game.
"I think he understands responsibility everywhere," he told the Buffalo newspaper. "You can't be everywhere all the time. So we didn't try to pressure him into anything. I think he understands responsibility, which is good. All of our condolences do go out to him and the family. He's got a lot on his plate, to say the least."