Donovan -- that Galaxy's all-time scoring leader with 92 goals -- helped lead Los Angeles to a second consecutive and fourth overall MLS Cup last year.
He was granted an extended leave of absence afterwards, skipping U.S. national appearances in the meantime while recovering from a grueling schedule in recent years.
Arena said Donovan has agreed to rejoin the team in late March, meaning he will miss the squad's first three regular season matches.
"After a series of collaborative discussions between Landon, his representative and our key club leaders, we have agreed to allow Landon to remain on an extended leave of absence and return to training with the team during the final week of March," Arena said.
"We understand this arrangement 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."