Napoli at least managed to grab a point away from home on Wednesday, better than their Serie A rivals and 2010 champions Inter Milan, who suffered a stunning 1-0 home defeat at the hands of Champions League debutants Turkish side Trabzonspor.
Trabzonspor were only in the group stage because rivals Fenerbahce were kicked out over a domestic match fixing scandal.
There were no such problems for other former champions Bayern Munich and Real Madrid as the Germans secured a fine 2-0 away win at Villarreal while Real beat Dinamo Zagreb 1-0 away.
Napoli took the lead with a brilliant effort as Edinson Cavani slotted home after a great run by Christian Maggio only for Aleksander Kolarov to level minutes later with a superb freekick.
City manager Roberto Mancini said that the occasion had perhaps got to his players.
"Maybe some players wanted to win the game alone, because we played one touch too many in the first-half.
"We wanted to win but the Champions League is hard."
United also started well against Benfica but last season''s finalists failed to take their chances, whereas Paraguayan striker Oscar Cardoso made no mistake for the hosts to break the deadlock. Cardoso controlled Nicolas Gaitan's delightful pass and fired home from the edge of the box.
United levelled just before half-time through the evergreen Ryan Giggs as he cut in from the right and fired home from outside the box - to set the bar ever higher for the oldest player to score in the competition as at 37 years and 289 days he beat his own mark set in April.
The Welsh great also set another benchmark making it 16 Champions League campaigns he has scored in, breaking the mark he shared with former Real Madrid icon Raul.
United manager Sir Alex Ferguson said that Giggs''s goal had settled them down.
"Once we scored, we played well," he said.
"Our possession was too cautious in the first half. It was the kind of possession that gives you a certain control but doesn't win you the game."
Inter just could not break down the Trabzonspor defence and the hosts were left stunned when Czech right-back Ondrej Celustka gave the visitors the lead in the second-half.
Trabzonspor's former Turkey national coach Senol Gunes said the difference had been that his players were not intimidated by their star studded opponents.