Gilmore had not been on the team's injury report after participating fully in practice throughout the week.
Although Gilmore participated fully, he has been experiencing headaches since the team's Oct. 5 win over the Tampa Bay Buccaneers, a source tells ESPN's Josina Anderson. Gilmore banged heads with wide receiver Mike Evans on a play, a source tells Anderson.
In addition to Gilmore, the Patriots have already ruled out No. 3 cornerback Eric Rowe (groin). So that leaves Malcolm Butler, second-year player Jonathan Jones and core special teams player Johnson Bademosi as the only other cornerbacks on the roster. The team has been using safeties Devin McCourty and Patrick Chung in some corner-type roles in recent weeks.