Giggs, who celebrates his 40th birthday on Friday, produced another superb display of his long and outstanding career in Wednesday's 5-0 romp at the BayArena.
Sami Hyypia's men simply had no answer to the Welshman's class, as some of their players just wondered how it was possible.
"I've run out of things to say about Ryan," Rooney said.
"Actually, during the game, the Bayer centre-half was asking how he is still playing at that age.
"I certainly won't be playing at that age, that's for sure. His composure on the ball is great."
With Giggs still capable of performing at such a high level, he might not even get his birthday weekend off given United visit Tottenham on Sunday needing to get their Premier League campaign back on track.
Rooney will not be getting the weekend off. Not after his four assists in what turned out to be United's biggest away win in Europe since 1964, and a victory margin they have exceeded only once on their European travels in their entire history.
"It's a team game but it's always great when you can help your team-mates score goals," Rooney said. "It was a perfect away match.
"We played very well, we defended well, there was some great counter-attacking football, we played with power, pace and composure.
"It was really as good an away performance as you can get."
Rooney also had words of praise for Shinji Kagawa, who revelled in his central position after being given the rare opportunity of operating behind a main frontman as he prefers.
United have now booked their place in the last 16, although they do need a point when they face Shakhtar Donetsk at Old Trafford in a fortnight to ensure they progress as group winners.
Before that is the clash with Tottenham at White Hart Lane against a Spurs side still smarting from their humiliating 6-0 defeat at Manchester City on Sunday.
Yet United can hardly afford to be charitable after they conceded a last-minute equaliser to drop two points at Cardiff a couple of hours later.
"Obviously, it was disappointing to lose a goal towards the end at Cardiff (on Sunday)," Rooney said. "We didn't play as well as we can.
"So, whilst we are all delighted to put on a great performance in the Champions League, now we are looking forward to playing Spurs."