London - Manchester United and Manchester City must switch their focus from the Champions League to the Premier League when they meet at Old Trafford on Sunday in a derby that Louis van Gaal says will show whether United are true title contenders.
United face City after labouring to a 1-1 draw with CSKA Moscow in the cold Russian capital on Wednesday while City make the short trip across town basking in the glow of their late 2-1 home win over Sevilla.
United must be ready because their manager Van Gaal believes a win for his side is vitally important.
"We have to be more consistent so next week when we win against Manchester City then we can say 'OK, now we think maybe about title aspirations'," the Dutchman said after last week's 3-0 win over Everton.
However Arsenal could be top for the first time in nearly two years by the time the two Manchester clubs meet on Sunday if they beat Everton on Saturday evening.
Arsenal have been in electrifying form recently with three successive wins including a 3-0 victory over Manchester United and a 2-0 Champions League win over Bayern Munich, and now face an Everton side, eager to put last week's defeat by United out of their minds.
With nine matches played, City top the table with 21 points, followed by Arsenal and United on 19, with West Ham United a further two back on 17.