Sir Alex believes Wayne Rooney will eventually break Sir Bobby Charlton's all-time United goalscoring record of 249.
After his hat-trick against Arsenal in August, Rooney admitted he'd set himself a challenge to reach Charlton’s incredible mark.
The number 10 is currently on 156 goals for the Reds, having scored nine in his first six league games this season, and Sir Alex sees no reason why he can’t surpass Sir Bobby.
The United boss told Inside United: "He's only 25 years of age so you'd think he has at least another five years in him at the very top level.
"And, if he looks after himself like Ryan Giggs, he could play well into his 30s. So the opportunity's there for him and time is on his side.
"He's raced out of the blocks already this season and I hope he could get up around the 30 mark."
Read the regular monthly interview with Sir Alex in the latest edition of Inside United , out on Wednesday.