Juan Mata says Manchester United face being caught by Liverpool if they do not end their scoring drought.
United suffered their third straight defeat on Saturday when they lost 1-0 to West Brom at Old Trafford.
United had 80 per cent possession, but could not find a way through Albion's rigid defence.
When Robin van Persie was presented with a chance to score from the penalty spot, he fluffed his lines and his effort was saved by West Brom goalkeeper Boaz Myhill.
Louis van Gaal's team have not scored in their last three games - and Mata admits they have to rediscover their killer touch in front of goal if they are to prevent fifth-placed Liverpool from bridging the four-point gap that separates them from their old rivals.
"The weekend wasn't good for several reasons," the United playmaker said.
"Obviously the 0-1 at Old Trafford is a tough score for us and we are forced to win as soon as possible: our rivals ahead keep adding points, and also the teams behind are getting closer.
"It's the tendency of the last few weeks: we have more ball possession than our opponent and create chances but we aren't able to take them.
"We must focus on being effective because we haven't scored in the last few games and we can't afford it any longer. On Saturday we tried until the end but their goalkeeper was brilliant."
A few weeks ago, United were being tipped to launch an 11th-hour title challenge, but those hopes are now over.
Chelsea lifted the trophy on Sunday after they beat Crystal Palace at Stamford Bridge.
Mata is happy for his former club, but he wants to lift the trophy with United next season.
The Spaniard added: "Chelsea are officially the Premier League champions. I had congratulated my former team-mates already and I do it again: you have been the most consistent team this season. I hope the congratulations are to us the next year."
Meanwhile, Spanish media are reporting Real Madrid are still hopeful of signing United goalkeeper De Gea, but say United value the 24-year-old at €40m.
United still expect the Spanish international to sign a new £200,000-a-week deal, but it is throught he will wait until the season ends before making a decision.
Van Gaal is also keen to send winger Adnan Januzaj on loan to PSV Eindhoven to secure 21-year-old Memphis Depay this summer.
United face stiff competition from Lancashire rivals Liverpool, with the player's agent saying a move to Anfield would be "great" for the striker.
However, Van Gaal remains the frontrunner for the Dutchman's signature having managed the player at international level.
United have also been boosted by the news that Brazilian youngster Andreas Pereira has rejected Paris Saint-Germain in favour of a new contract at Old Trafford.
"I am pleased to have signed my new contract," Pereira said.
"I would like to thank the manager for giving me the opportunity to join the first-team squad. I made my Premier League debut against Tottenham Hotspur and to play at Old Trafford in front of so many fans was a dream come true.
"I am at the right club to continue my development as a player and to learn as much as I can. I want to thank my family and God for the opportunity he has given me."