Mourinho warned of a "difficult" season when he wasn't backed in the transfer market before the deadline for Premier League clubs closed before the opening weekend of the season.
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.
David de Gea mans the goal for the visitors.
Goal! Shane Duffy smashes the ball into the bottom corner from close range! Our second goal came too late.
"We thought, I think it was before the game against Basel, Pogba is a young guy, with a good personality, made in the academy, normally a starting line-up player, so he has the conditions to be a captain in the future."
It didn't come, but it didn't matter.
"We've spoken about United but we've also got to give Brighton some credit as well".
It's not quite that bad for Mourinho but his preseason prediction is already coming true.
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Woodward failed to deliver such targets like Jerome Boateng, Toby Alderweireld, Harry Maguire and Diego Godin, leaving Mourinho frustrated with the current centre back options he has available.
Strengthening the heart of his defence was Mourinho's priority and it showed as Victor Lindelof and Eric Bailly endured a torrid afternoon on England's south coast.
Lindelof was at fault for the first, letting Murray steal in to score.
Sanchez, 29, did not travel with the team on Saturday afternoon after picking up a problem in training and is now a doubt for Tottenham's visit to Old Trafford on August 27.
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. Matic underwent surgery for an injury he picked up at the World Cup whereas Valencia limped off in United's match against San Jose Earthquakes with a calf strain.
There are problems at the other end, too, and United managed just three shots on target while Lukaku was limited to just 24 touches all game. Brighton finished 15th a year ago after being promoted, but the real test is this season. The Iranian global was the top scorer in the Eredivisie last season and enjoyed an impressive World Cup for Team Melli.
Only four teams managed to win at Brighton last season and it will be a hard place to play this season, too.
The Red Devils have always maintained that they have no intention of parting with a man who has been a model of consistency for them during a testing period - with that form recognised with a record-breaking haul of four Player of the Year awards.
If they can do that more often than not this season, they should win enough home games to stay in the league for a third successive campaign.