The Spaniard came on for the second half but only managed six minutes before being forced off with a painful knock to the ribs.
Scott McTominay replaced him and was one of the seven players to miss in the following penalty shootout, which Man United eventually won 2-1.
And Mourinho, speaking after the game, couldn’t confirm for certain whether the 27-year-old had picked up a muscle injury but acknowledged that it was painful.
“At least very painful, because for him to come out it has to be painful. I want to wait but I say to the players no risk in friendly matches, if you feel something, that something is coming, a muscular injury, get out, the result is not important,” he said.
“But he was very painful, maybe muscle, I don’t know.”
Herrera has, funnily enough, suffered rib damage before in his United career, and detecting their severity from witnessing just the mere impact can be difficult.
At best, the affliction is just a nasty contact injury that will naturally clear up over a few days, but there is always a chance the Spaniard may have to wear a brace and miss a crucial part of pre-season.
And following what was a ponderous second half from United in the midfielder’s absence, supporters will pray to see him back in action soon.