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.
West Ham's credentials will come under the spotlight when they face champions Chelsea, who are 11th having won just three of their nine games so far, one of those being last week's 2-0 home win over strugglers Aston Villa.
Two managers will also come under even more intense scrutiny than usual -- Juergen Klopp, who will enjoy his first Premier League home match when Southampton visit Anfield and Sam Allardyce, in charge of his first home match as Sunderland's boss when they take on his old club Newcastle United in the Tyne-Wear derby.
Klopp, whose first game ended in a 0-0 draw at Tottenham Hotspur last week, will make his home debut against Rubin Kazan of Russia in the Europa League on Thursday, something of a dress rehearsal before Sunday's main act.
Allardyce, who briefly played for Sunderland in the early 1980s and managed Newcastle for eight months eight years ago, has a huge task ahead saving the bottom club from relegation.
Getting their first win of the season against their arch-rivals would be the perfect place to start after his first match ended in a 1-0 defeat at West Bromwich Albion last week.
Seventh-placed Spurs currently boast the longest unbeaten run in the league having gone eight games since their only loss, a 1-0 defeat at Manchester United on the opening day.
They face Anderlecht in the Europa League in Brussels on Thursday then travel to Bournemouth, crushed 5-1 at Manchester City last week without three key long-term injuries, Callum Wilson, Tyrone Mings and Max Gradel, all out for the season.
The clubs have only ever met once before when Bournemouth, then in the third tier, beat top flight Spurs 3-1 in an FA Cup fifth-round match in February 1957.
A win for Bournemouth, who have slipped to 17th, on Sunday would not be quite as big a shock but it would still be a surprise.