'If you want to be super critical and look at the players he's got. "So we had to decide to give an opportunity to another attacking player".
"I think it's going to be a very hard season", he said in an interview on Friday.
Mourinho also hailed Lindelof as the best technical defender within the squad while also highlighting that the player had already given good performances in the 2017-18 season, albeit in European competitions.
And while United were City's closest rivals for the title last time out, Mourinho's men finished some 19 points behind the Etihad outfit.
Lindelof was at fault for the first, letting Murray steal in to score.
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. "Not my ideas, it's the players who make the tactics".
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. This was an improvement on their display in last season's 1-0 home win over the same opponents and will give the manager hope of a top-half finish this term.
Unless you've been living in Pyongyang the past few weeks you might have noticed that champs Man City have a documentary out which captured the drama of the expensively-assembled side's romp to the Premier League title last season. I repeat, I want him to play well, I want him to play for the team, the fans, and that is what it is.
Mourinho also said Brighton's early goals had robbed United of their "confidence".
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.