Sanchez did not train with the United squad on Monday, after which Mourinho confirmed he would play no part at Old Trafford.
“The players you saw [training] are the players who are available,” Mourinho said. “Nothing to hide. Alexis is out.”
Sanchez came on as a late substitute in the recent 2-2 draw with Chelsea at Stamford Bridge.
The Chilean had arrived back in London straight from international duty last week, after playing 90 minutes in a friendly for his country in Mexico.
Sanchez had joined up with his United team-mates in the capital, rather than travelling up from Manchester.
Mourinho also addressed captain Antonio Valencia’s omission from recent match-day squads.
“The reason was that he was 10 days without training, not an injury but he had a very complicated surgery on his mouth and he needed time to recover his level after that.”