Take a look at the sentence again:
The problem occurs between recommend and a. He feared the treatment the doctor would recommend is a complete sentence. A week's worth of bed rest during the nicest week in April would certainly wreck Jim's golf plans is the next complete sentence. Because these two parts have just a comma between them, they create a comma splice, a major sentence error.
To fix the problem, you could put a period after recommend and then capitalize the A.