The midfielder has been plagued by a serious tendinitis problem in recent seasons and has had to undergo operations on both knees.
He has been rehabilitating in America over the summer but is expected back at United's Carrington training complex on 23rd September.
The Red Devils start their Champions League campaign against Besiktas on 15th September and Sir Alex Ferguson has insisted he will not rush Hargreaves back into action.
But having included the 28-year-old in their European squad, United will hope he can play some part later on in a group that also includes Wolfsburg and CSKA Moscow.
In total, United have registered 35 players for the competition, a number swelled by 10, including Anderson and Jonny Evans, who were born after 1st January, 1988 and have spent two years at the club.