She has muscles, believe me. Boxers build muscle strength from repetition (like hitting the punching bag for years) not from weightlifting. Boxers are generally lean and not muscle bound.
I am a boxing fan and it doesn’t concern me in the least that she isn’t showing ‘muscles’.
I can tell from the trailers that Kim So-hye has been doing some serious training for this part.
I mentioned in another comment sometimes in sports dramas there is not that much ‘sports’. We will have to see. Presumably because she was away from the ring for three years (an eternity in sports) she will have a lot of training cut out for her to get back in shape. That is probably where a lot of the ‘sports’ stuff in the drama will occur. Just speculating.
I am a boxing fan and Lee Sang-yeob fan. That is why I will be watching.

I don't mind an adaptation changing a story, especially if it means changing problematic aspects. The power imbalance between coach and athlete bothers me, especially with the big difference in age and life experience implied by casting a (real-life) 24-year-old woman with a 40-year-old man. The show will have to work hard to convince me that it's not an exploitative relationship. I really like Lee Sang-yub and would like to see him as a romantic ML, but this may not be the right vehicle.

And I'm with @kafiyah-bello in wondering how those stick-like arms can pack enough of a punch to knock down a hamster, let alone the tall, muscular man in the preview.