United manager Alex Ferguson called it "the tie of the round," and Real Madrid director Emilio Butragueno said it would be "thrilling for everybody."
In another blockbuster pairing in Thursday's draw, Champions League favourite Barcelona and Lionel Messi will play seven-time winner AC Milan.
Arsenal will face Bayern Munich, last season's beaten finalist, and big-spending Paris Saint-Germain was paired with Valencia.
Also, it was: Celtic vs. Juventus, Shakhtar Donetsk vs. Borussia Dortmund, FC Porto vs. Malaga, and Galatasaray vs. Schalke.
First-leg matches are to be played from Feb. 12-20. Return matches are from March 5-13.
The storied history of nine-time European champion Madrid and three-time winner United would be enough to ensure their matchup top billing, with the first leg in Spain on Feb. 13.
The friendly rivalries of Mourinho and Alex Ferguson in the dugout, and star forwards Ronaldo and Wayne Rooney on the pitch add an extra dimension.
"It's a great opportunity for our fans to see Cristiano again and also for me to meet up with Jose again," Ferguson said in comments posted on the club's official website.
The coaches met in Manchester this month, ahead of Madrid's group match against English champion City, adding to speculation that Mourinho covets the United job.
"(Mourinho) is a fantastic manager, a fantastic coach and of course this tie is going to be a challenge for all of us," Butragueno said, declining to discuss whether the reportedly unhappy Portuguese will leave Madrid after the season.
It was at Old Trafford where Mourinho truly arrived on the international scene with a flamboyant victory sprint down the touchline as his Porto team eliminated United in the last 16 in 2004, the first of Ronaldo's six seasons in England before joining Madrid.
"It is going to be very special for Cristiano," said Butragueno of the Portugal forward, who is the competition's top scorer with six goals. "He is an extraordinary professional. I am sure he is going to do his best to help us qualify."
United defender Rio Ferdinand wrote on Twitter: "Oh yes, Madrid!! What a great couple a games that'll be!! (at)Cristiano see u soon bro!!"
Milan had the misfortune to draw Barcelona again after being unable to beat the Catalan side in four matches in last season's group stage and quarterfinals.
"They're not pleased, we're not pleased but this is what the destiny is," Milan director Umberto Gandini told The Associated Press with a smile. "We have to go over the most difficult team on the planet."
Barcelona director Amador Bernabeu said the matchup, which opens in Milan, would be a meeting of two "important, classic teams."
Hours before the draw at UEFA headquarters, Barcelona coach Tito Vilanova underwent surgery on his throat and faces weeks of chemotherapy and radiotherapy treatment.
UEFA general secretary Gianni Infantino opened the ceremony with a message to "a friend of ours. Tito, we are all with you."
Barcelona hopes Vilanova will be on the bench at the San Siro on Feb. 20, Bernabeu said.
Bayern chairman Karl-Heinz Rummenigge said the runaway Bundesliga leader "can be happy" with facing Arsenal, which has struggled this season.
"We can manage this, we're going as favourites there," Rummenigge said. "It's important that we try to get a good result in London, to score a goal and set down a marker."
Shakhtar looked like a team to avoid, and German champion Dortmund — which won a group including Madrid — must go to Ukraine for the first leg on Feb. 13.
"They finished first in a very strong group," Shakhtar chief executive Sergei Palkin told the AP. "Our overall goal is to win the Champions League."
Paris Saint-Germain and talismanic forward Zlatan Ibrahimovic will start as favourite against Valencia, the Spanish club's director Jordi Bruixola said.
Celtic coach Neil Lennon, who masterminded a group-stage victory against Barcelona, said the Scottish champion would respect the European tradition of two-time winner Juventus.
"In terms of glamor it's great, in terms of qualification it's going to be very tough," Lennon told British broadcaster Sky Sports.Suggest a correction