Jose Mourinho's side need still eight points to secure the league title from their previous six matches, but Mata is convinced his former employers have now effectively wrapped it up.
The Spain international admits he was "angry" about the defeat, "thrilled" after seeing old friends and was proud of his side's performance despite the defeat. He also admits the reception of the Chelsea supporters moved him.
Mata moved by reception and congratulates Chelsea
"It was really moving to feel the people’s respect and affection, and I will always be grateful for that," Mata wrote on his latest blog. "To them and also to the players. I thank them for such affection and I congratulate them for the title they are about to win, because it’s almost done.
"It was nice to meet them and chat with them after the game, and also the club’s staff, doctors, physios, etc., and feel their respect and friendship. Thanks to all and congratulations."
Eden Hazard scored the only goal of the game late in the first half to extend Chelsea's lead over second-placed Arsenal, who were in action against Reading in the FA Cup on Saturday, to 10 points.
Mourinho angered by Terry's comment
Mourinho, however, is taking nothing for granted and told reporters after the United win he was angry with John Terry after his captain allegedly said the Blues now 'felt like champions'.
Asked if he felt Chelsea now had the Premier League title wrapped up, Mourinho was quoted by the London Evening Standard as saying: "No - and I'm not happy if John said that."
Chelsea, though, are now the overwhelming favourites to win the Premier League title for the first time since 2010 - with the majority of bookmakers pricing them as short as 1/150 to lift the trophy within the forthcoming weeks.
The Blues face Arsenal next weekend, before fixtures against Leicester City, Crystal Palace, Liverpool, West Brom and Sunderland on the final day of the season.
No bad blood between Mata and Chelsea
It's clear that no bad blood exists between Mata and his former employers or Mourinho, who only used the attacking midfielder sporadically before selling him to Manchester United in January 2014.
“He's a player with a history in this club,” Mourinho was quoted as saying by the London Evening Standard before Saturday's match. “Every player in this club has deep respect for him as a player, as does the manager.
“We all wished him the best when he left. We are all happy with him: a very correct boy, total professional. We're happy things are going well with him.”
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