Gilmore has undergone surgery on the wrist and is expected to return to the team within eight weeks, a league source told ESPN, putting his possible return at Week 6 when the Bills play the Cincinnati Bengals.
Gilmore left in the second quarter of Saturday's preseason loss to the Washington Redskins with what the team announced as a hand injury. Marrone said the injury will require surgery.
The team will not, however, use the injured reserve/designated to return tag on Gilmore, which would have kept him out at least the first eight weeks of the regular season.
Gilmore is Buffalo's top cornerback. The 10th overall pick in the 2012 draft, Gilmore finished with one interception in his rookie season but was considered one of the Bills' best defensive players.
ESPN Senior NFL Insider Chris Mortensen contributed to this report.