Two minutes later they doubled their lead when, after one ball had fizzed across the box, United failed to deal with the second and Duffy turned and fired into the corner.
Lukaku, who missed a golden chance at 0-0, nodded in a lifeline for his side but more slack defending saw Eric Bailly bring down Gross, who dispatched his penalty a minute before the break.
The defeat means champions Manchester City have already stolen a march on their neighbours in the title race, after they thrashed Huddersfield Town 6-1 earlier on to make it two wins from two.
His goal was a quite superb first-time finish and he generally ran Bailly and Lindelof ragged, allowing the home side plenty of reprieve from any United pressure during the second half.
And ahead of today's trip down to Brighton (4pm), Mourinho toldSky Sports: "We were speaking about one extra body, it's not like we were unhappy because we wanted six or seven more players and they didn't come".
Lee Grant will be United's back up keeper as Sergio Romero is still out.
DC mayor says she got through to Trump on parade
An extravagant military parade planned by President Trump for November in Washington DC has been put on hold until next year. Earlier this year, the White House budget chief said the parade would cost US taxpayers between $10 million and $30 million.
Who will come out on top in the battle of the managers Chris Hughton or José Mourinho? It was a 1-0 win over United here in May that secured their top-flight status last season and this could turn out to be just as significant.
The documentary series, titled All or Nothing: Manchester City, was released on Friday and was met with glowing reviews by those looking to get a glimpse into the reality of being a Premier League footballer. That is my first reaction.
It wasn't a surprise when Mata didn't return for the second half.
In team news, Mourinho should welcome back Romelu Lukaku & Jessie Lingard after their World Cup adventures.
Summer signing Fred impressed on his Premier League debut against Leicester.
Souness was on punditry duty for Sky Sports when he delivered a brutal assessment of the Red Devils as he insisted that the current crop of players is the least impressive since before Sir Alex Ferguson took over at Old Trafford.