Louis van Gaal believes defeat to Manchester City in Sunday's derby would end Manchester United's hopes of Champions League qualification this season.
United head into the match four points behind City, who will begin the weekend in fourth place and keen to damage their rivals' prospects of a return to Europe's top competition.
Van Gaal saw his hopes of continental glory extinguished for another year by Liverpool in the Europa League on Thursday and is clear on the consequences of coming up short against another local foe.
"I think so [defeat would leave too big a gap]," he said.
"We are now four points behind I think so you have to win, otherwise the gap is bigger and bigger and the matches that you have to play are not so many.
"It will be seven points and we will have eight matches to play. It would be very hard to recover from that gap.
"I think it would be difficult then because West Ham United are also in front of us, so it's not only City, but West Ham United."
Van Gaal's standing with United fans has continued to fall over recent weeks, with former players such as Paul Scholes and Rio Ferdinand establishing themselves as vocal critics of the team's performances and playing style.
Jose Mourinho has been regularly linked to his post, but the Dutchman is happy to continue under the assumption he can complete a three-year contract that expires in June 2017.
"I'm never feeling under pressure, I know what I have to do," he added.
"I have a three-year contract and the process is three years. Our aim is to reach the top three because we want to do better than last year [a fourth-place finish in 2014-15]."