Sunday saw United's six-game unbeaten run in all competitions come to a spectacular end as Chelsea beat them 4-0
Sunday saw United's six-game unbeaten run in all competitions come to a spectacular end as Chelsea won 4-0 in the Premier League at Stamford Bridge.
It was a return to forget for both former Chelsea manager turned United boss Jose Mourinho and midfielder Mata, who played for the London club before the Portuguese froze him out of first-team duty and then sold the Spain midfielder to United.
United were behind after just 30 seconds when an error allowed Mata's compatriot Pedro Rodriguez to open the scoring and from there it was largely one-way traffic with Gary Cahill, Eden Hazard and N'Golo Kante all adding further goals.
But Mata, a half-time substitute, said United would be glad of Wednesday's League Cup clash at home to City as this would give them a chance to atone quickly for the Chelsea loss.
"It sounds like a cliche but it's true: when we win and everybody praises you it's easy to be united and celebrate together, but it is now, after such a defeat, when we must remain mentally strong and keep going," Mata said.
"Together we win and together we lose."
Despite the fact United were well beaten, their fans never let up in their vocal support for their side -- so much so that Chelsea manager Antonio Conte was seen urging the home crowd to match the volume of the visiting support following the Blues' fourth goal.
"Thank you so much to the fans that came to London, you didn't stop cheering us on through the 90 minutes, even when things became so difficult. You guys are amazing," said Mata.
"And also to the Chelsea fans, for they always show me their respect and affection.
"We hope to give you the win that you deserve this coming Wednesday."
That match will be the second derby clash since both Mourinho and City manager Pep Guardiola arrived in Manchester.
City won 2-1 at Old Trafford last month despite a less than convincing club debut by goalkeeper Claudio Bravo.
Having won their first 10 games under Spanish boss Guardiola, Sunday's 1-1 draw at home to Southampton means Premier League leaders City have now gone five games without a victory.