The 18-year-old Adu, who had scored his second goal for Benfica in the Portuguese Cup over the weekend, and 19-year-old Argentine Angel Di Maria entered the game on the hour mark.
The Portuguese club pounded the Celtic but was denied by Polish goalie Artur Boruc until Di Maria's chip found Paraguayan Oscar Cardozo, who chested the ball down and poked it past Boruc from eight yards out with three minutes remaining for Benfica's first win after two losses in group play.
At 18 years, 4 months, Adu broke the record for the youngest American to play in the Champions League held by Jonathan Spector (18 years, 11 months) set when he came on as a sub for Manchester United against Turkey's Fenerbahce in 2004.
Adu had also played for Benfica in the qualifying stage of the Champions League.
Oct. 24 in Lisbon
Benfica 1 Celtic 0.Goal: Cardozo 87.
Benfica -- Quim, Luisao, Leo, Pereira, Katsouranis; Costa, Assis (Di Maria, 61), Rodriguez (Luis Felipe, 84), Binya, Cardozo, Bergessio (Adu, 61).
Celtic -- Boruc, Caldwell, Kennedy, McManus, Naylor, Jarosik, Brown, Hartley, Donati (Sno, 63), McGeady, Killen (McDonald, 74).
Referee: Massimo Busacca (Switzerland).