Pep Guardiola’s men are 13 points clear at the top of the table with just 12 games remaining.
But despite Mourinho’s comments, Valencia insists United must keep on hoping the gap can be bridged.
Back in 2012 United blew an eight point lead in the final six games to let City pip them to the title and Valencia knows anything can happen.
“We lost a title when we were eight points ahead and there were only a few games left. Now, we have a lot more games to play and we need to keep fighting,” Valencia told the club’s website.
“We need to keep going. “The important thing is to get the three points, playing at home, playing well, and we will carry on with this mentality until the end of the season.”
United bounced back from a 2-0 defeat by Tottenham Hotspur last Wednesday to beat Huddersfield Town 2-0 on Saturday.
“We have showed that we have the quality and conditions to fight until the end of the season,” Valencia added. United travel to 16th-placed Newcastle United on Sunday.