Rojo, like recently re-signed striker Zlatan Ibrahimovic, underwent surgery in May after sustaining knee ligament damage during a Europa League quarter-final second-leg win over Anderlecht at Old Trafford and the initial prognosis was that he might not play again in 2017.
The versatile centre-back attended the Europa League final on crutches and later continued his rehabilitation in the US, where United travelled for their annual pre-season tour. He also accompanied his teammates to Skopje for the Uefa Super Cup clash with Real Madrid.
Mourinho always maintained that Rojo would not be available again until December or perhaps even January, although hopes of a potential early comeback grew last week when the Red Devils posted pictures of him back on the training pitch.
However, while it seems that his recovery is progressing very well, it appears that there has been no change with regards to that aforementioned timescale.
"There is no limping, there are no details of injury, but that is just body language, that is just straight running. Football is different, we have to respect the timings and the timings are bringing Marcos to the team around December, not before."
Rojo and Ibrahimovic are United's only current long-term injury concerns, with Mourinho still expecting the latter to be in contention for the second half of the season despite suggestions that he could be in the frame as early as October.
Ashley Young and Luke Shaw have yet to play competitively this season due to respective groin and foot injuries suffered towards the end of 2016-17, although both came through 58 minutes of an under-23 meeting with Swansea City at Leigh Sports Village unscathed on Monday evening.
Mourinho believes the former will return to senior duty first during a busy September post-international break schedule that features four Premier League fixtures, two Champions League matchdays and a Carabao Cup third-round meeting with Championship outfit Burton Albion.
"Young will be earlier than Luke," he said. "There are seven matches to play in September in three different competitions, sometimes just two days between matches, so we need our squad. We need everyone."