Sanchez missed Arsenal's 4-1 league win over Crystal Palace with manager Arsene Wenger saying that his long-anticipated departure was dependent on the Armenian agreeing to join the Gunners.
"Sanchez (transfer) will only happen if Mkhitaryan comes. That's why it takes longer," the Frenchman said, adding: "I expect it to happen. In the next 48 hours it will be decided."
Mkhitaryan joined United from Borussia Dortmund in 2016 but appears to have fallen out of favour with manager Jose Mourinho, having not completed a full 90 minutes in the league so far this season.