Thursday’s match is set to see United midfielder Paul Pogba, 23, go up against his older brother.
Florentin Pogba, 26, is poised to line up in Saint-Etienne’s defence at Old Trafford.
He marked the “magical” encounter against younger brother by having the numbers 19 and six shaved into his hair – their squad numbers.
“The 19 is my squad number and the six is for him,” said Florentin. “It’s going to be a magical moment and I hope he and I enjoy it the best we can.”
Their mother, Yeo, will be at the first leg and next Wednesday’s return fixture in France, enjoying the “one-off” occasion to the maximum.
Mourinho said: “I think mother Pogba is in trouble, which is normal.
“It is not difficult for the lady to choose, it is impossible. I know from Paul that his mum wants a draw.
“But next week, one son will be happy and another will be sad. But only for their mum is it a problem.
“For them, I think they will enjoy playing against each other. Paul is excited. It is a good thing.”
Saint-Etienne are fifth in Ligue 1, 19 points behind leaders Monaco.