Needing to avoid defeat to have any chance of making it out of Group C Jorge Jesus' side had to wait until the 82nd minute in Lisbon before the young striker lashed home a left-wing corner.
Nicolas Gaitan was particularly lively in the first half, and, having been unmarked in the penalty area, saw a late effort superbly blocked by the Monaco defence.
Jardel, replacing the suspended Lisandro Lopez, was one of four changes for the hosts from the sides' meeting two weeks ago, and they made a positive start to proceedings.
Monaco, who gave Lacina Traore his first start in the Champions League in place of the injured Dimitar Berbatov, found themselves on the back foot for large parts of the first half, but were grateful that Benfica's finishing lacked accuracy.
Talisca, who had scored eight in his last nine Benfica matches, then went close at the other end, but his powerful shot on 51 minutes was pushed away by Subasic.
As the hour approached Monaco began to pressurise the Benfica goal, and they would have been ahead had Julio Cesar not made a fine save to deny Ferreira-Carrasco before Luisao bravely blocked Traore as the Ivorian burst clear.
Gaitan's corner was flicked on by Derley and Talisca benefited with a low finish under the body of Subasic from inside the six-yard box.
