1940’s. Harold believes he is a baseball star and proves it in the minor leagues only to have circumstances send him in another direction.

"circumstances send him in another direction." sounds too vague to me.

I always recommend writers not to make their antagonists faceless. The logline may be stronger if we know who/what exactly Harold is up against.

Also 'another direction' is another vague concept. Can you specify?

The whole point of the logline is to test that these specifics are strong enough for a screen story. It shouldn't be too hard to work them into one sentence.

Finally: 'proves it in the minor leagues'... So what tells us he is NOT the baseball star he believes he is?