It looked like the Portuguese manager would need to utilise it early on as well after Danny Graham opened the scoring after 17 minutes, although his blushes were saved somewhat by a Marcus Rashford equaliser not long after.
The bench featured the world’s most expensive player in Paul Pogba as well as Juan Mata, Eric Bailly and Luke Shaw who all cost more than £30m each.
Bastian Schweinsteiger was a slightly less expensive acquisition at £6.5m and Zlatan Ibrahimovic arrived on a free transfer – although both players could have attracted astronomical fees at the height of their careers.
The only player not worth millions of pounds was 19-year-old goalkeeper Dean Henderson.
Nevertheless, the total cost of the subs bench was an eye-watering £197.5m.
Mourinho made seven changes to the team that beat St Etienne on Thursday night.
Michael Carrick, Matteo Darmian, Jesse Lingard, Henrikh Mkhitaryan, Rashford, Marcos Rojo and Ashley Young all started.
Manchester United bench v Blackburn
Paul Pogba: £89m
Juan Mata: £38m
Eric Bailly: £32m
Luke Shaw: £32m
Bastian Schweinsteiger: £6.5m
Zlatan Ibrahimovic: Free
Dean Henderson: Academy
David De Gea was rested again, with Sergio Romero continuing in goal. Wayne Rooney sat out injured.