1 DEFENCE SUSCEPTIBLE TO PACE
The absence of the athletic Joel Matip through injury means that Liverpool's central defensive pairing of Dejan Lovren and Ragnar Klavan, while physically imposing and commanding in the air, is vulnerable to passes over the top to pacy forwards.
Southampton's intention to turn Klavan was clear early on, when Virgil van Dijk pinged a long diagonal over the Estonian's head in search of Jay Rodriguez.
For the goal, Klavan was caught too high up the pitch, allowing Nathan Redmond to steal in behind the offside trap and score.
Mourinho must instruct Marcus Rashford to dart in behind the defence, and Anthony Martial's pace means that he could have a role to play too.
2 LACK OF WIDTH IN WIDE AREAS
Neither Roberto Firmino nor Adam Lallana are natural wingers, and neither started the season there - Firmino began the campaign as the No. 9 with Lallana on the left of the midfield three before the absence of Jordan Henderson and Sadio Mane forced a switch.
So, both are more comfortable cutting in from the flank, and at times that worked in their favour, no more so than when Lallana's back-header teed up Firmino.
But their instinct to come central and link up also has its downsides - it meant that Liverpool become congested in the middle (particularly with Daniel Sturridge's predilection to drop deep) and are left sorely lacking the natural width of Mane. Mourinho will be happy if United can funnel Liverpool's attacks into central positions, particularly with the central defence in excellent form.
3 COULD FIND JOY DOWN THE RIGHT
At times in the first half, James Milner was Liverpool's most advanced player. As a converted midfielder, his instinct as a full-back is to stay high up the pitch, particularly when play is down the opposite flank, so he stretches the defence and creates holes for Lallana and Firmino to scamper into.
He is adept at finding space and providing an outlet so faltering attacks are often recycled through him.
However, his inclination to attack, as well as Firmino's reluctance to track back, allow the opposition right-back space to raid forward, as Cedric Soares did. He started the move that led to Redmond forcing Loris Karius into a good save before the goal.
As a former winger, Antonio Valencia should exploit this.
4 HOLDING ROLE PERMUTATIONS
Oriol Romeu had a superb game for Southampton patrolling in front of the back four.
At times the Spaniard stuck to Firmino; at times he ranged further forward, shackling Emre Can and Georginio Wijnaldum and cutting off the supply to Firmino and Lallana at source; and at times he simply held the line in front of the defence, forcing the man in possession to play a sideways ball.
It will not be straightforward for Mourinho to copycat that role though - he has finally found a midfield three that works for him and none of Michael Carrick, Ander Herrera or Paul Pogba is a natural destroyer.
It would be a brave move to pick Daley Blind or Marouane Fellaini.
THE TIMES, LONDON