Holders Man Utd and Villarreal reach last 16

(CNN) -- Manchester United and Villarreal advanced to the knockout stages of the Champions League on Tuesday after a 0-0 draw in Spain.

Both clubs advanced after improving to nine points in Group E, with Aalborg ending Celtic's interest in Europe with a 2-1 comeback win in Denmark.

Villarreal goalkeeper Diego Lopez starred again, saving a handful of chances from Cristiano Ronaldo, including two dangerous attempts in the first half, as United extended their unbeaten run to 18 games.

The match was the fourth straight scoreless draw between the two clubs.

"We deserved better -- we improved our possession in the second half and I was satisfied," United manager Alex Ferguson told ITV1. "It takes us into the next stage and that's the main thing."

Asked whether he felt Villarreal had set their stall out to claim the point that would see both sides into the last 16, Ferguson replied: "They were happy to get a draw."

The Spaniards finished with 10 men after Joan Capdevilla was sent off after 82 minutes for a foul on Portugues winger Ronaldo.

Scottish hopes Celtic crashed out when Gary Caldwell's late own-goal giving the Danish side the points.

Andreas Johansson sent a cross into the area in the 87th and Glen Looven's attempted clearance hit Caldwell, who deflected it into his own net for Aalborg's winner.

Barry Robson had put Scotland's Celtic ahead on 53 minute by heading in a cross from Mark Wilson. Caca equalized for the hosts with a powerful low shot in the 73rd minute past goalkeeper Artur Boruc.

United had clear second-half chances from Wayne Rooney and Darren Fletcher but could not crack Lopez who had also been stellar in a scoreless draw at Old Trafford.

Lopez got a finger up in the 41st minute to tip Ronaldo's swirling drive from outside the area off the bar after Rooney had laid it up perfectly. Two minutes later, Lopez was diving to his right to push the Portugal international's curling free kick out.

Capdevilla headed Rooney's angled header off the goal line in the 79th and was expelled two minutes later for a knee-high lunge on Ronaldo.

United enjoyed the better run of play over the final 10 minutes with Carlos Tevez and Park Ji-sung brought on as substitutes with Villarreal content to escape with a draw after Capdevila's sending off.

Caldwell insists Celtic only have themselves to blame after they were sent spinning out of Europe.

"Having got in a position to win the game and thrown it away like that, (it was) very disappointing," the Scotland defender told STV.

"We haven't been good enough on the night and we haven't been good enough in the campaign. We have to deal with that and we have to get better from it. We've got try and learn from it.

"We've been in the last 16 for the last two years. We know how good it is. We want to be there. We've paid the price for some sloppy performances."