Giggs, who will turn 38 on November 29, cancelled out Oscar Cardozo's fine 24th minute goal with an equally well-taken finish three minutes before halftime for last season's beaten finalists.
"This is why Giggs is such a legend, why he has been one of United's best players for so many years," said United captain Patrice Evra.
United manager Sir Alex Ferguson was happy enough with a point.
"All in all, it was a good European night, good to watch. I thought the two goals were terrific," he told a news conference.
"I thought our possession in the first half of the game was too cautious. Once Ryan scored we played much better."
Ferguson rang the changes for the Lisbon clash, naming an inexperienced back line and counting on the likes of Giggs and Park Ji-Sung in the middle while leaving Nani and Javier Hernandez on the bench. Ferguson also opted for back up keeper Anders Lindegaard in place of David De Gea.
Benfica threatened first when lively winger Nicolas Gaitan volleyed a diagional shot just wide after a clever lay-off from Paraguay striker Cardozo in the 17th minute.
The two combined again to create the opening goal seven minutes later, with Gaitan sending in a fine pass from the left that the striker chested down to skip away from Jonny Evans and fire in past Lindergaard. United had not conceded a goal in their previous six Champions League away games.
United created little in the opening half with in-form striker Wayne Rooney unable to make an impact.
But they conjured an equaliser just before the break with their first effort on target, Antonio Valencia finding Giggs on the edge of the box and he fired a rasping drive past Artur.
"Benfica had a great game, we played eye-to-eye against United, creating a lot of chances and they scored with their first chance on target," said Benfica midfielder Javi Garcia.
Giggs almost grabbed a second on 64 minutes, dribbling past three defenders to shoot from near the penalty spot, but Artur made a reflex save to tip the ball away for a corner.
Benfica responded two minutes later with the game's best flowing move, Aimar leading a counter attack and his compatriot Gaitan serving substitute forward Nolito, who placed a shot that Lindegaard had to stretch to palm away.
Growing in confidence, Benfica drove forward and Spaniard Nolito shot wide when clear on goal with just three minutes left.
"United are the big favourites in the group. Every point counts and one at home against United is not great but it is good," said Benfica coach Jorge Jesus, who had earlier suggested he would be satisfied with a draw.
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