Donovan struggled with fatigue for much of 2012 but kept playing to help Galaxy retain their Major League Soccer title in December.
The American international has not played since and will not be back in time for the USA's crucial World Cup qualifiers against Costa Rica and Mexico next month.
Galaxy coach Bruce Arena confirmed in a statement today that he has allowed Donovan to report late for pre-season training.
He said: "After a series of collaborative discussions between Landon, his representatives and our key club leaders, we have agreed to allow Landon to remain on an extended leave of absence.
"We understand this agreement is not ideal, but we are confident it will allow Landon the time he needs to be both mentally and physically prepared for the upcoming season."
The 31-year-old will miss Galaxy's first six games in all competitions.