Trescothick last week pulled out of next month's Champions Trophy in India, saying he needed more treatment for a stress-related condition which had also cut short his tour of India in February.
''As someone who has had his problems with being away from home playing for England, I feel for Marcus,'' fast bowler Harmison told The Mail on Sunday newspaper.
''I have been warning about the pressures, stresses and strains of being away from home playing cricket, in England as well as abroad, and have been criticised for speaking openly about homesickness.
''But people have got to understand we are not robots. We are normal human beings with families and lives back home.
''It gives me no pleasure to say this, but with the amount of time we are away from home and the amount of cricket we are playing, there is going to be a price to pay and a casualty.
''Somebody at the height of his powers is going to prematurely lose his career, not through injury but mental stress. I just hope and pray it's not going to be Marcus.'' Despite missing the trip to India, England selectors are expecting Trescothick to be fit for the Ashes tour of Australia starting in November.
Harmison, who said he had suffered ''big time'' from stress two years ago in South Africa, looked on the positive side of Trescothick having flagged up his problems before the Ashes.
''Had we known he was struggling the last time in India, we might have been able to do something for him. By the time he'd gone, it was too late.
''This time if he starts to wobble we can rally round him, lift him and give him all the support we can,'' Harmison said.
''Sure, the Aussies are going to have a go at him, but they will be having a go at everybody, so what's new?'' Trescothick was out to the first ball of Tuesday's one-day defeat by Pakistan in Southampton and scored only 22 runs in the first two games of the series.