NYON, Switzerland: Barcelona versus AC Milan and Manchester United against Real Madrid were the pick of the ties in the Champions League last 16 draw on Thursday, as the knock-out phase of this year's competition took shape.

Perennial favourites Barcelona, champions in 2009 and 2011, are currently nine points clear in La Liga but received a hammer blow on Wednesday when it was revealed coach Tito Vilanova must undergo surgery after cancer returned to his salivary gland.

His assistant, Jordi Roura, has been placed in temporary charge.

Barcelona and Milan locked horns four times in last season's competition, with Barca finishing above Massimiliano Allegri's side in the group phase and then prevailing 3-1 on aggregate when they met again in the last eight.

United's tie with Madrid will see Portugal skipper Cristiano Ronaldo face his former club for the first time since leaving Old Trafford for the Spanish capital in 2009.

The sides last met in 2003, when a famous hat-trick in Manchester by Ronaldo -- Cristiano Ronaldo's Brazilian predecessor and namesake -- gave Madrid a 6-5 aggregate quarter-final win.

Madrid coach Jose Mourinho has only lost two of his 14 previous encounters with Alex Ferguson's side, and United club secretary John Alexander admitted the club would have preferred a more gentle draw.

“Everyone wanted to keep this for later in the competition but, out of several stand-out ties, I think this is the one,” he told British television channel Sky Sports News.

“If history has anything in our favour, our one victory against Real Madrid in this competition was in the season when we won the final at Wembley (in 1968).”

Former Madrid player and club director Emilio Butragueno said: “I'm sure it's going to be a special game for Ronaldo.

“He's a great professional so I'm sure he's going to do his best to help Real Madrid qualify for the next round, but going back there will be special for him.”

Arsenal were drawn against last season's beaten finalists, Bayern Munich, who have constructed a nine-point lead in the Bundesliga after a storming start to the season.

“We know Arsenal well,” said Bayern chairman Karl-Heinz Rummenigge.

“That's fortunate and we can be happy about the draw. The game won't be a foregone conclusion and we should not under-estimate Arsenal.”

Meanwhile, Celtic's reward for reaching the knockout phase for the first time since 2007-08 is a rendezvous with Italian champions Juventus.

“In terms of glamour, it is great,” Celtic coach Neil Lennon told Sky Sports News.

“In terms of qualification it is going to be very tough. But we will take it as it comes and our supporters will be looking forward to this match.”

Big-spending French club Paris Saint-Germain will take on Valencia, with German champions Borussia Dortmund slated to face their Ukrainian counterparts Shakhtar Donetsk.

In the remaining ties, competition debutants Malaga tackle Porto and Turkish outfit Galatasaray meet German side Schalke.

The first-leg matches will be played on dates between February 12-13 and February 19-20, and the return legs between March 5-6 and March 12-13.

Champions League last 16 draw: Galatasaray (TUR) v Schalke 04 (GER) Celtic (SCO) v Juventus (ITA) Arsenal (ENG) v Bayern Munich (GER) Shakhtar Donetsk (UKR) v Borussia Dortmund (GER) AC Milan (ITA) v Barcelona (ESP) Real Madrid (ESP) v Manchester United (ENG) Valencia (ESP) v Paris Saint-Germain (FRA) FC Porto (POR) v Malaga (ESP)