The Hammers confirmed a £650,000 deal for the youngster on deadline day, after a troubled time at Old Trafford.
Morrison was allowed to leave United after a series of off-field problems that resulted in various fines and court appearances and the 19-year-old saw his route to the first-team blocked because of his consistent poor behaviour.
"Ravel is a talented lad and if he really wants to be a player then we will help him as much as we can along the way," said the West Ham manager.
"Perhaps a change of environment will be beneficial to him and help him make that step up to first-team football quicker.
"There is no doubt he has exceptional talent."
Morrison made three first-team appearances in his time at the Red Devils.