The Red Devils suffered another setback in the top flight on Saturday as they slumped to a 1-0 defeat by Southampton at Old Trafford.
That result means that Louis van Gaal’s men have now only won two of their last 10 Premier League games.
United, who have not won the title since 2013, are currently fifth in the table and five points behind fourth placed Tottenham Hotspur.
However, despite their recent struggles Sanchez is wary of the threat posed by United, as well as Chelsea and Manchester City.
Sanchez is quoted as saying by the Daily Mirror: “I am very surprised by Chelsea’s results this season and by the exit of Jose Mourinho. They are one of the biggest clubs in the league, but this year aren’t in the race for the title. But I’m sure they will beat some big teams because their pride is enormous.
“Every game is difficult and the title race is close. Leicester have been a surprise and we know about Manchester City.
“But Manchester United also have a great team and are always candidates for the title too.
“They have changed their coach this season and it takes players time to get used to a new system, but I’m sure they will be tough opponents in the title race. So, it’s important we keep going.”
Sanchez is bidding to help Arsenal win their first Premier League title since their famous unbeaten triumph in the 2003-04 season.
The Gunners won the FA Cup in the Chilean’s first season at the club last term.
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