Having opened his account in Tuesday's UEFA Super Cup loss to Real Madrid in Macedonia, Lukaku now has three goals in two United games. Manchester United have a few players out as well, with the knee injuries to Marcos Rojo and James Wilson still keeping them out long term, as well as long waits off the pitch left for Luke Shaw and Ashley Young.
Mata was through again shortly after following a lovely looped pass over the top by Pogba, but the Spaniard's square pass for Lukaku at the far post was cut out.
And he then doubled United's lead seven minutes after the restart, heading Henrikh Mkhitaryan's free kick into the bottom corner.
Mata's darting runs into the box were bringing United joy and he nearly broke the deadlock when he raced onto Pogba's clever dinked pass and slipped a shot past Hart, only for Angelo Ogbonna to hack clear.
Eric Bailly and Phil Jones will start in the heart of the defense, while Antonio Valencia and Daley Blind play in a flat back-four as Mourinho decides to ditched to the three-man backline. It's been coming and it had to be Lukaku on his debut.
Javier Hernandez was there as the rot set in under David Moyes and he will make his Old Trafford return on Sunday after joining West Ham from Bayer Leverkusen in a £16 million deal. He slips in a attractive ball to Lukaku and the Belgian slips it past Hart at his near post.
And the Argentinian paid a heavy price for that foul for, when Mkhitaryan floated over the consequent free-kick, that man Lukaku ghosted in front of Masuaku to nod home his second goal of the afternoon. Red Devils fans will be optimistic for an assault on the Premier League title and it could turn into reality, providing they manage to transfer some of their home draws into wins.
"If the pass is on, the top players will find you". Great from Pogba, awful from Hart.