The Red Devils' manager was banished to the stands by referee Craig Pawson during the final seconds of the match at St Mary's Stadium for "encroaching onto the pitch," per Mark Jones of the Daily Mirror.
Mourinho could now be in line for disciplinary action, either in the form of a fine or touchline ban.
Jones described how Mourinho incurred the referee's wrath as he "maniacally tried to get his players back into position to see off the late threat of the hosts."
As Samuel Luckhurst of the Manchester Evening News noted, Pawson was alerted to Mourinho's actions by a member of his officiating team:
Luckhurst also gave his own take on what the United chief had done:
For his part, Mourinho claimed confusion over the reasons behind his sending off, although he did admit to colliding with the fourth official, per Luckhurst: "I don't know. I don't know. Craig told me to leave and I left. I don't know. He told me to leave, I left."
Mourinho is no stranger to getting into trouble with match officials. In his report, Jones also cited how the 54-year-old received a ban and a fine worth £58,000 for disciplinary infractions during league matches against Burnley and Liverpool last season.
If another ban is forthcoming, United could chance an appeal given Mourinho's confusion about why he was sent to the stands. If not, assistant Rui Faria will take Mourinho's place in the dugout—a familiar stand-in role for the United No. 2.