United fell 15 points behind runaway leaders Manchester City with a 2-0 defeat at Spurs on Wednesday that also saw the gap between United in second and Tottenham in fifth cut to just five points in the race for Champions League qualification.
Sanchez has yet to score in two appearances since joining from Arsenal in a swap deal that saw Henrikh Mkhitaryan head to the Emirates.
However, midfielder Juan Mata believes Sanchez's mere presence has helped raise the bar of what can be expected of United for the rest of the season.
"His arrival is great news because a player of his level always raises the competition within the team and our competitiveness."
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United are set to be without Marouane Fellaini who was substituted against Spurs just eight minutes after he had come off the bench as a substitute.
Mourinho confirmed the Belgium midfielder has a knee ligament injury and joins defender Eric Bailly (ankle) and forward Zlatan Ibrahimovic (knee) on the sidelines.
- 'Decisive spell' -
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And Mata has warned his team-mates not to underestimate David Wagner's side even though the Terriers are winless in seven league games.
"Huddersfield are struggling to win in this start of 2018 but they already showed in our first game against them what they're capable of," added the Spanish playmaker.
"We need to try to learn from that experience so we can make the most of our strengths and get the three points.
Huddersfield midfielder Philip Billing urged his team- mates not to be intimidated by the prospect of facing United's stellar names in front of nearly 75,000 people at Old Trafford.
"It's Manchester United and they have world class players but you can't think about that - it's 11 versus 11," said the Denmark Under-21 international.
"Anything can happen in football, we beat them at home and I see no reason why we can't beat them again if we take our chances."
"It's frustrating for us at the minute but that's football, it's now that tests you as a team," added Billing.
"Now we've got to show that we're the terrier and we've got the fighting spirit.