Wayne Rooney has cast doubt on Arsenal's Premier League title credentials.

After extending their unbeaten run to eight matches in all competitions with Tuesday night's goalless Champions League draw with Real Sociedad, Manchester United's focus is now trained on Sunday's showdown with Arsène Wenger's men.

Despite negative pre-season expectations around a club that has won nothing since 2005, Arsenal have shot out of the blocks.

The arrival of Mesut Özil – the man Rooney advised Sir Alex Ferguson to sign – from Real Madrid appears to be a masterstroke and, with all their rivals making a faltering start, Arsenal have already established a five-point cushion at the top.

But Rooney is remaining cautious about their threat. There have been plenty of times in the past when Arsenal appeared capable of mounting a sustained challenge, only for it to peter out when the pressure starts to be applied later in the campaign.

"We'll have to wait and see where Arsenal are in March," he said. "We've seen before that they've been in the top two until February or March and then faded away. They are doing brilliantly at the minute, so it's down to them to try and stay there and us to catch them."

After losing three of their opening six games, United have been gradually clawing their way back into contention. Despite registering back-to-back league victories for the first time this season, United remain in a rather unaccustomed eighth position. But the gap to all but Arsenal, who lead them by eight points, has been closed to a more manageable level.

However, after collecting a single point from their early-season meetings with Chelsea, Liverpool and Manchester City, United have something to prove about their own durability, beyond the simple requirement of victory.

"We're obviously capable of beating Arsenal," said Rooney. "We've done it many times in the past. They're on a good run of form at the moment and they're top of the league, so we know it's going to be a difficult game.

"But we have to go into it with confidence and a belief this is when we can edge closer to them and bring us back into the mix of being at the top of the league."

While the fallout from Tuesday's Champions League stalemate concerned Ashley Young's conduct in winning a penalty, Robin van Persie's failure to convert it and Javier Hernández's earlier inexplicable miss, the fact remains, in Europe, United are unbeaten and still on course to reach the last 16 as group winners. In contrast, domestic progress has been a tortuous process. Indeed, it could be argued given their overall series of results, United are somewhat fortunate not to be totally cut adrift already.

It was a point made by Gary Neville in the aftermath of Chelsea's defeat at Newcastle last weekend when the former United defender claimed everyone was waiting for United to make their move. "When you know one of your rivals' results before the game – especially if they've been beaten – it's great to feel you can edge closer," said Rooney. "We know we're capable of playing better and we're trying to do that. It's up to us to keep trying to pick up points and then, come Christmas, see where we are."

Nevertheless, if any club is capable of emerging from their present position as champions, it is the one Rooney remained at despite extensive overtures from Chelsea during the summer. The 28-year-old's impressive form has already proved there is no lingering disappointment at not getting the move to Stamford Bridge he apparently craved.

David Moyes's work in talking round his one-time Everton protege means United can harbour ambition that might already have been extinguished had Rooney not remained at Old Trafford.

"We had a whole new setup and different ideas from the manager," he said. "It was up to us to try and implement them. It was a bit of a change but we're starting to show the things that the manager wants.

"There are only nine or 10 games gone so there's no big worry at the moment. We're starting to do well. We're confident at the minute, we're getting more clean sheets and playing with freedom. If we can get the victory on Sunday then we're in a great position."