I hope they don't let him push himself too hard, and I don't think they will. Athlete's are competitors and can be stubborn though.
With any young pitcher you want their innings pitched to go up in a regular progression. Add chemo to that and I wouldn't push him beyond what he did last year.
Besides, Lester can use the work with his control. I hope they tell him early on that he's going to pitch 5-6 inning outings in Pawtucket for at least the first 3 months and then be promoted to Boston in July or August regardless his performance. Tell him that even if he's throwing 6 innings of no-hit ball every start in Pawtucket he's not coming up, and even if he gives up 6 runs in every start he's still coming up.
That would free him up to work on his control and command of his pitches. It would also allow him to come back from chemo and learn to deal with the ongoing therapy.
They'd have him at the end of the year, when his contributions would matter the most, and set him up to contribute in '08. Lester is still young and the Sox would be better off with him strengthening himself this year (both physically and as a pitcher).