London - Ryan Giggs lit the fuse on Manchester United's potentially explosive clash with Chelsea by claiming the Premier League leaders aren't the finished article yet.
While Chelsea have made a flying start to their title challenge with seven wins from eight matches to open up a four-point lead over second-placed United, Giggs believes it is too early to crown the west London club as champions elect.
Chelsea's 2-1 defeat at Shakhtar Donetsk in the Champions League on Tuesday underlined Giggs' belief that the rebuilding job undertaken by Blues boss Roberto Di Matteo after the departure of several senior stars is still a work in progress.
"Chelsea are a talented team. They have changed the personnel a bit with Oscar, Hazard and Mata, who play in those little pockets and are so hard to pick up," Giggs said ahead of Sunday's clash between the top two at Stamford Bridge.
"Only time will tell if they are better. But it doesn't really matter because you know you are up against a very good team when you play Chelsea, and it will always be tough at Stamford Bridge."
United would dearly love to win a league game at the Bridge for the first time since 2002 as they try to close the gap on Chelsea, but to do that Giggs knows they must find a way to cure their defensive woes.
Alex Ferguson's team have conceded the first goal in eight matches out of 12 this term and they were at it again on Tuesday when Braga stunned Old Trafford by taking a 2-0 lead in the Champions League before United hit back to win 3-2.
"There are two ways of looking at it," Giggs said. "Obviously we are pleased that we are able to come back and have shown great character in those eight games.
"But you cannot keep relying on the lads up front to score the goals. It has happened again against Braga but against Chelsea if we go behind it will be tough to come back."
Meanwhile, Chelsea goalkeeper Petr Cech is confident his team's slip-up in Donetsk will prove a valuable lesson ahead of the United match.
The Blues have lost four of their last five meetings with United, but Cech expects a strong response after the European setback.
"It is good that this happened now because Manchester United is a similar team who play on the counter-attack away from home," Cech said.
"We will play more or less the same kind of side as Shakhtar. We didn't deal with it well so hopefully this will serve as a lesson for the weekend."
Third-placed Manchester City will also look to get back on track after a 3-1 defeat at Ajax in midweek left their Champions League challenge hanging by a thread.
The English champions have spluttered so far this season and City midfielder James Milner wants his team to be more ruthless against Swansea at Eastlands on Saturday.
"We weren't clinical enough against Ajax," Milner said. "We need to make sure there is not pressure on the strikers to score in every game. Goals have got to come from all over the park."
Like City, Arsenal, who host bottom of the table QPR, are in need of a morale-boosting win following successive defeats against Norwich and Schalke.
"We have great players with great potential but at the moment you cannot feel we are a strong squad," Arsenal defender Per Mertesacker said.
"We have to learn quickly from what happened in the last two matches.
"If we do not find these solutions, it is going to be difficult both in the Premier League and in the Champions League as well."
On Sunday, Liverpool manager Brendan Rodgers gets his first taste of the Merseyside derby as the Reds make the short trip to Goodison Park to face Everton.
Everton midfielder Marouane Fellaini hopes to recover from a knee injury in time to feature and his teammate Kevin Mirallas said: "It's good to have Marouane available because he is scary for the other team. People in the opposition fear him."
Arsenal v QPR, Aston Villa v Norwich (1145GMT), Man City v Swansea (1630GMT), Reading v Fulham, Stoke v Sunderland, Wigan v West Ham
Chelsea v Man Utd (1500GMT), Everton v Liverpool (1230GMT), Newcastle v West Brom, Southampton v Tottenham