Ifill limped off in the first half of the opening-day clash at Home Park, leaving Millwall boss Dennis Wise with a selection headache for the forthcoming home games against Wigan Athletic in midweek and Leicester City next Saturday.
Wise told the club's official site: "Their player caught him on the back of the calf - it wasn't a clever challenge.
"We'll have more idea of how it is on Monday but he will certainly miss the next two games, if not longer."
New signing Josh Simpson is expected to make his first start for The Lions on Tuesday in place of Ifill.