Young among Manchester United absentees for Valencia
The England international picked up a problem in the 3-1 weekend defeat at West Ham and will miss Tuesday's match along with Ander Herrera, Jesse Lingard and long-term absentee Marcos Rojo.
Losing 33-year-old Young is a fresh blow to Mourinho's hopes of setting a stable base, the United manager having unsuccessfully shifted to three central defenders at London Stadium.
Diogo Dalot, who impressed on debut against Young Boys, is now expected to vie with Antonio Valencia for the right-back berth.
Lingard is recovering from a groin injury, while Herrera - an unused substitute against the Hammers - was battling an ankle problem prior to his return in the EFL Cup penalty shootout defeat to Derby County.
"Apart from Marcos Rojo, now we have Ander Herrera, Ashley Young and Jesse Lingard out," Mourinho told MUTV.
"I prefer the doctor to speak, but they are not in [the right] conditions."
The Valencia clash could prove pivotal to United's hopes of advancing from Group H with two matches against Juventus still to come.
"It's a big week," Jose says. "Everyone wants to play in the Champions League, so we are here and we have the chance to play a big match." #MUFC #UCL
— Manchester United (@ManUtd) October 1, 2018