The England full-back joined the Red Devils from Southampton in 2014 amid much fanfare, but form and fitness issues have restricted him to just 48 appearances for the club.
Shaw returned to the fold last term but Mourinho publicly questioned his attitude and application during a comeback campaign ended in late April by foot ligament damage.
Although the 22-year-old came on as a half-time substitute in United's 4-1 win over Burton Albion in the EFL Cup on Wednesday, Mourinho insists that Saturday's game against the Saints has come too early for the player.
The Portuguese coach told reporters: "He has to work. You saw the [Burton] game, you saw the same as I see, so are you asking if he plays [against Southampton]? No, he is not playing. He has to work. He has to improve.
"Look, he doesn't play for six months. I am not expecting him to arrive on Wednesday and to be man of the match or to be running up and down for 45 minutes. It's just a process. He doesn't play for a long, long time.
"You have other players in the Premier League that in this moment who are not starting matches and their managers just say 'they're not in the best conditions'. And we're speaking about some of the best players of the Premier League. I heard last week the Chelsea manager saying that, I heard Arsenal manager said that.
"Luke had surgery and a long time without playing football. I cannot expect him to be back and to be strong, strong, strong in his work."
Shaw made just 11 Premier League appearances in the 2016-17 campaign.