Darren Fletcher believes this season will present Manchester United with their 'biggest test' in trying to win the Premier League.
United secured a record-breaking 19th title last season and have made an imperious start to the defence of their crown after winning their opening four matches.
However, local rivals Manchester City have matched that achievement and Chelsea are just two points further adrift after three wins and a draw.
Chelsea made a blistering start to last season and face United on Sunday having seen their title dreams ended at Old Trafford back in May.
Fletcher, who is now back to fitness after a virus, expects a tough challenge against Chelsea and also believes old adversaries Liverpool will be in the mix for the title.
"I think Chelsea have been overlooked," said Fletcher. "Chelsea started the season on fire last year, but didn't win the league, so we have to be careful.
"Chelsea are a top side with a lot of experience. They have a new manager and they have brought younger, fresher and exciting players in.
"Liverpool have signed players as well, so it's a tough league to play in this season.
"Although the two Manchester clubs have started well, the other teams are going to be dangerous.
"With the quality of the teams in the league this year, I think it will be our biggest test so far to go and win the title."