Knockaert was brought down 25 yards out, getting up to curl the free kick over the bar, but three minutes from the break Brighton had a penalty when Eric Bailly fouled Gross as he broke through - and the Brighton man blasted the kick in off De Gea.
Mourinho has pinpointed the usual suspects as United's chief threats but claims that Tottenham, who became the first Premier League side ever to not make a single summer signing since the introduction of the transfer periods, had the most successful window.
Jose tried to make changes at half-time to inspire a comeback but the team failed to immediately respond, or at least in the first minutes of the second half.More news: Donald Trump says will attend November WWI parade in Paris
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United came back from two goals down to win twice last season and when Romelu Lukaku headed in his first goal of the season from close range it looked like it might be on again.
Paul Pogba's penalty in second-half stoppage time was too little, too late for disjointed United.
His positioning was generally poor, he conceded a needless corner from which Brighton scored their second, while his rash challenge on Gross gifted them the third from the penalty spot.
Despite losing here last season, United are favourites to secure three points and despite the loss of Sanchez, will be expected to keep their winning start to the season alive. Just three months down the line, the Seagulls were at it again, with Gross - who hit the victor against United last term - instrumental again.
Manchester United fans were definitely disappointed by the weak performance that was put in by the players vs Brighton. Or was last season a fluke? The left-back took part in extra training sessions before preseason in order to get himself fit for the 2018/19 campaign so he will be desperate to retain his place. Are they good enough to stay in the top flight?