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 wrote on his blog.
"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 wrote: "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 ;-)."
Source : PA
Source: PA