The Red Devils have made a promising start to the Premier League season following a return of 16 points from their opening six games.
Manchester United were 1-0 winners against Southampton at St Mary’s last weekend thanks to Romelu Lukaku’s 20th-minute winner on the south coast.
Mourinho’s men have netted 17 times and have conceded two goals so far but Manchester United trail their derby rivals on goal difference.
Manchester City were 5-0 winners against Crystal Palace at The Etihad last weekend to take their haul to 22 goals in the 2017-18 season so far.
Former Manchester United midfielder Scholes warned Jose Mourinho’s side that being able to defend would be the decisive factor in the title race.
“I think it will come down to who can be better defensively – who can concede the least goals,” Scholes told Metro.
“I don’t think either side [Manchester City or Manchester United] has really been tested yet against a top team. When they do we’ll have a better idea.”
Manchester United are looking to win their first Premier League title since 2013 in Mourinho’s second season in charge at Old Trafford.
The Red Devils signed Benfica defender Victor Lindelof, Chelsea midfielder Nemanja Matic and Everton striker Romelu Lukaku in the summer transfer window.
Mourinho’s side will take on Crystal Palace at Selhurst Park in their next Premier League fixture on Saturday.
MORE: Man United latest news
MORE: Arsenal latest news
MORE: Chelsea latest news
MORE: Liverpool latest news