The United manager woke up at the team hotel on the morning of Saturday's huge Premier League clash, only to find out that his selected starting XI was being circulated on social media.
The furious Portuguese is now set to launch an internal investigation to solve the mystery of the leaked lineup, to ensure such a thing never happens again.
Only players and club staff were privy to the starting XI; which Mourinho had announced to them the day before, and despite being widely photographed travelling from Manchester to London that day, there was no way that the lineup could have been leaked by anyone else other than those involved with the United first team squad.
It is the second time that the team has been leaked by a member of the club in recent weeks, as the same happened on United's last trip to the capital, an embarrassing defeat at the hands of West Ham United last month.
Now, after the incident occurred in the lead up to United's clash Chelsea at the weekend, in which Mourinho felt his former club could have been handed an advantage, the Portuguese is adamant about exposing the culprit.
Prior knowledge of the inclusion of Juan Mata demonstrated to Sarri and his Chelsea assistants exactly how the United team would plan to set up and approach the match.
The leak almost did not matter for United, as in spite of a poor first half performance, Mourinho's side took the lead and looked as though they would take all three points. Only to be thwarted at the death by a Ross Barkley equaliser.
The disappointment of dropping points after such a valiant performance was too much for Mourinho; irked by celebrations of Chelsea assistant Marco Ianni, he prompted a touchline scuffle between the two benches at the very end of the match.
United will now have to refocus, as they face a tough midweek Champions League tie with Juventus and Cristiano Ronaldo at Old Trafford on Wednesday, which they need to get a result from, if they hold any hope of qualifying from their tough group.