Dareus has been excused from Bills practice to be with his family after his brother's death.
"I talked to him this morning," Gailey said Wednesday, according to WGR-AM. "I think he'll be back for Sunday's game, whether it's tonight, tomorrow or the next day I don't know. But I think he'll be here for this week. He wants to be back."
Dareus' brother was one of three people shot and killed in a suburb of Birmingham, Ala., last weekend.
Gailey expressed his sympathy to Dareus and his family.
"All of our condolences do go out to he and the family," Gailey said. "He's got a lot on his plate to say the least."
Dareus' younger brother, Simeon Gilmore, was one of three people shot and killed during a burglary in Pelham, Ala., on Sunday. Authorities said 20-year-old Jon Staggs Jr. has been charged with capital murder and burglary.
Police have not released a possible motive.
Dareus, Buffalo's 2011 first-round draft pick out of Alabama, traveled home earlier this week to be with his family.
The Associated Press contributed to this report.