You can get the machines into cells: white blood cells demonstrate that systems of molecular machinery can move through tissues. Viruses demonstrate that systems of molecular machinery can move through cell membranes to enter cells. The mobility of organelles inside cells demonstrates that systems of molecular machinery can move around inside the cell. The fact that cell biologists can stick needles into cells and do surgery on chromosomes and sometimes have the cells survive shows that things can enter cells and do even very crude manipulations without doing permanent damage in many cases. So you can get repair machines to the site of the damage. You can identify, take apart, and put back together molecular structures. If the overall process is orchestrated [by] a computer, then it seems you have everything necessary to repair cells.
This image shows cell repair underway, using molecular repair devices.
K. Eric Drexler, "Molecular Technology and Cell Repair Machines," paper presented at the 1985 Lake Tahoe Life Extension Festival, 25 May 1985; reprinted and published in Claustrophobia Magazine (August-October 1985) and in Cryonics Magazine (Dec 1985 - January 1986).