United looked set for a comfortable win after moving 3-1 ahead at the King Power stadium on Sunday, but some abject defending in the final half hour allowed Leicester to record their first home win over the Red Devils in 29 years.
Rafael and Tyler Blackett both gave away penalties and slack marking allowed Leonardo Ulloa and Esteban Cambiasso to find the net in a shocking meltdown which cost United the match.
Mata was at fault for Jamie Vardy's goal as he surrendered possession to Ritchie De Laet, who cantered up the field to set up the Leicester striker for the hosts' fourth.
In his weekly blog, Mata wrote about the deep pain he felt at being part of the defeat, which leaves United 12th in the Barclays Premier League after five matches.
"After a whole week waiting for the match I could not imagine a Sunday like this," the United forward said.
"The way back from Leicester to Manchester on the bus seemed really long.
"I keep thinking about how we lost a game that was under control.
"It's really tough to score three goals away and come back home with no points.
"On our way back to Manchester I was thinking about what had happened and felt so disappointed as all those that came with us to the stadium.
"Honestly, I feel very sorry that the weekend ended up for our fans in such a sad way."
Mata, who came on for Radamel Falcao in the second half, said he had spent "a few hours" trying to come up with the words to describe his disappointment at the defeat.
Others were quicker to pass judgement
Former United captain Gary Neville said the team looked "soft-centred" and ex-Holland striker Ruud Gullit criticised manager Louis van Gaal for spending the majority of his transfer budget on the likes of Falcao, Angel di Maria and Ander Herrera when defensive reinforcements were needed more
Former United coach Rene Meulensteen also expressed his surprise that Van Gaal had not yet handed Luke Shaw his debut.
Van Gaal left Shaw on the bench for the last two games despite a number of injuries in defence.
Asked whether he thought his fellow Dutchman does not rate £28million signing Shaw, Meulensteen told TalkSPORT: "Yes
It's a big surprise for me.
"He would have been informed by the club that they were putting this bid in for Shaw because he's an up and coming player.
"He's got a good left foot and a lot of pace
He's got a lot to learn to get to a Man United level though.
"He's a great prospect but he's brought (Marcos) Rojo in, which is bad news for Shaw."
It seems highly likely that Shaw will make his first appearance against West Ham next week.
Jonny Evans suffered a foot injury on Sunday and Blackett will be absent through suspension after his red card at Leicester.
With Phil Jones also sidelined due to a hamstring injury, Van Gaal looks like he will have no option but to move Rojo to centre-back alongside Chris Smalling, with Shaw starting at left-back.
Midfielder Marouane Fellaini looks to be making progress in his return from an ankle injury.
The Belgium midfielder, who has been out since the opening day of the season, posted on his Instagram page on Sunday night: "Back to Manchester tonight, #ready #training #tomorrow'."
Mata, meanwhile, hopes United will be able to bounce back from the Leicester defeat by recording back to back wins over West Ham and Everton before the international break.
"There is time during the week to analyse what happened and learn from our mistakes so as not to make them again," Mata added.
"Now we will play two consecutive games at Old Trafford that we want to win for our fans
We will put in our best efforts, you can be sure of that.
"I hope to be in a better mood next Sunday."
Source : PA
Source: PA