Lingard missed United's 2-1 win over Leicester City last week having just returned from holiday following the 2018 World Cup, while Lukaku was only a second-half substitute against the Foxes due to his limited fitness.
Mourinho, however, has suggested that Lukaku and Lingard could both start the clash with Brighton this weekend.
"I think we will have the same players [as against Leicester]. I think the injured players aren't going to be back for Brighton," he told the club's official website.
"What we have is Paul [Pogba] with nine days' training, [Marcus] Rashford with 11 days' training, [Marouane] Fellaini with nine days' training and, of course, they will be in a better condition.
"Normally, these boys that [against Leicester] were in a difficult situation will be ready to give a bit more [against Brighton] – more Lukaku, more Rashford, probably Lingard coming back, more Fellaini. I think normally we're going to have more."
United suffered a 1-0 defeat when they travelled to Brighton in the Premier League last season.