After experiencing severe abdominal pain at the surgical site, he was sent to a panel of surgeons for guidance. Going into surgery at this time would've been catastrophic to his career, as after a surgery one cannot fly into space for some extended period of time. At this point in his career Hadfield was relatively green, and there was a plethora of new astronauts as talented as he was just waiting to take his spot. Going into surgery at this point would have relegated him to a successful career at mission control, but not in space.
To the best of my recollection, his physician's opinion was he didn't need a full surgery to investigate this abdominal pain. However, the panel of surgeons universally, unequivocally recommended the gastrointestinal surgery. He states infinitely more eloquently than I can how applicable "uf you only have a hammer, everything looks like a nail" in regards to the surgical panel.
I recall he decided against the recommendations of the surgeon panel, and decided his abdominals wouldn't suddenly tear apart with potentially fatal results while orbiting earth in the ISS. It turns out he guessed right, and had one of the most prolific careers of any astronaut of the last few decades.
While this anecdote may make it seem like I am some anti-surgeon, anti-surgery person, let me state that I have had one surgery (besides wisdom teeth, which "doesn't count" as it's so common). It was an orthopedic reconstructive ulnar collateral (thumb) surgery, and my repaired thumb is honestly almost indistinguishable from my left thumb. And in this case it was clear surgery was needed, but after reading Hadfield's book and the parent article here it was astounding how the surgeons were literally unable to see any mode of treatment which didn't involve surgery. I think just like overprescription of antibiotics, we as a society should look at the overprescription of surgeries for medical issues which can be treated without surgical intervention.