With two games to play, all four clubs in the group remain in contention for two berths in the knockout stage.
Benfica went into the game with one point and one goal, needing victory to keep its tournament hopes alive. Talisca delivered with a tap-in at the back post from a corner.
Monaco, struggling in front of goal this season following the departures of James Rodriguez and Radamel Falcao, was unbeaten before its trip to Lisbon.
Bayer Leverkusen leads the group with nine points and Monaco has five, one clear of Zenit St. Petersburg and Benfica.