Former Manchester City defender Danny Mills feels Pep Guardiola's Premier League leaders "almost look unstoppable" at the moment, but he stressed there should be no assumptions they will end up winning the title -- particularly with Jose Mourinho in the mix.
City moved eight points clear of Mourinho's second-placed United once again on Wednesday night as Raheem Sterling's stoppage-time goal earned them a 2-1 victory over Southampton.
Guardiola's side, who face United at Old Trafford on December 10, have won 13 league games so far this term and drawn their other one.
Speaking in Salford on Thursday at an event marking the first anniversary of the EFL's Every Player Counts project, Mills told Press Association Sport when asked about City: "They almost look unstoppable. They are playing so well. It is brilliant to watch, and that is down to Pep.
"But on the other half of the city you've got Mourinho, who is the pragmatist, the defensive mastermind. He is good at what he does, and he is a winner, simple as that. The derby is going to be incredibly fascinating.
"At the start of the season I went with Manchester United just to pip Manchester City [to the title] as I wasn't quite sure how City would react this season. New signings were coming in -- would they bed in? Would Guardiola's system and the way he wants to do things work?
"It clearly is. He is doing an absolutely outstanding job, and City are absolutely brilliant at the moment.
"But there is still that one caveat that Mourinho knows how to win, and he has been here before. He has won the Premier League, and knows how to dig in when the going gets tough.
"At the moment it is City's to lose - they are the in-form team and playing brilliant football. But Manchester United are not going to go away. We can't take anything for granted."