The Red Devils have not enjoyed the best of starts to the current season under David Moyes and would slip 11 points behind the Gunners with a loss at Old Trafford.
|15/2||Chelsea are 15/2 with BetVictor to beat West brom 1-0|
"It's on the edge [with a loss] but United is United," Mourinho told reporters.
"So even 10, 11 points... I wouldn't say it's over for them. If they lose... in this Premier League, I don't think it's impossible for them to be champions."
The Portuguese also commented on former Manchester United boss Sir Alex Ferguson's view that Mourinho's first stint at Stamford Bridge changed the way the Red Devils prepared their season.
In his autobiography, Sir Alex explained how before Mourinho arrived at Chelsea for the first time in 2004 United had sought to pace themselves over the course of the season, building up to a strong finish rather than bursting out of the traps.
"He [Ferguson] told me that," Mourinho added.
"He used some funny words, like saying the championship was a marathon but after [Chelsea] it became a marathon in a sprint. I know it's true that, after that, United felt that they needed a different approach.
"We pushed them to a different level than they were in 2004 and 2005, when it was quite comfortable for us to win our Premierships."