Manchester United's win over Liverpool FC lifted them up into second place in the Premier League table as they look to try and finish as close to champions elect Manchester City.
But Mourinho, who has won the competition twice before with Porto and Inter Milan, said United will start "smelling" the semi-finals if they manage to get past Sevilla.
"I think we're not one of the best teams in the competition".
Writing in his column for Sky Sports, Nicholas said: "This might not be as big for the fans, but this is as big as Saturday in terms of it being huge for them to be in Champions League quarter-finals. We are playing against a team that didn't lose against Liverpool home and away", he told reporters at his pre-match press conference. I say Liverpool because they are an example of a very good team, a good example of an important stadium with a big tradition.
Paul Pogba is a doubt for for the tie after the 24-year-old missed Saturday's Premier League win against Liverpool through injury and manager Jose Mourinho is unsure whether the midfielder will be available on Tuesday.
"So imagine if we were knocked out - it would be considered a disaster, but for me it would be a disappointment".
"It's fine, it's something you've got to accept so that is where I'm at".
"We don't have to win by three, four, five goals, we have to win 1-0, 2-1, 5-4, 8-7".
That victory meant the Red Devils secured a Champions League spot for only the second time since the retirement of club legend Sir Alex Ferguson, and Mourinho, who had dismissed the Europa League as a competition in the past, admitted how vital that triumph was.
Speaking to La Sexta, and covered by Marca and others, Mourinho was asked about De Gea potentially moving to Real Madrid in the summer transfer window.
The France worldwide missed Saturday's 2-1 win over north-west rivals Liverpool due to a thigh injury, but Samuel Luckhurst of the Manchester Evening News confirmed he was back in the squad ahead of Tuesday. "But he trained a little bit".
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