The United manager had pencilled in the midfielder for his first start of the season in Monte Carlo, but had to withdraw him after a flare up in Thursday evening's training session.
"He said he felt his knee and that he was not fit enough to play," Sir Alex confirmed. "There is nothing you can to do about it. If a player declares himself unfit you have to accept that."
Hargreaves was dogged by tendonitis throughout his first season at Old Trafford, and the Reds' boss admits he is keen for the combative star to get his Reds career back on track.
"We'd have liked to play him and get him back into the run of things and playing football again," he said. "To be honest, he has not played enough football in the last year. He'd like to get back and it is disappointing."