A former boxer’s last fight forced him to retire (due to brain injury). His estranged wife finds out she is pregnant by him and their unborn child needs surgery. Now he must return to the world of fighting (underground fight circuit run by a gang) – at great personal risk – to not only win the money to save their child, but to also win back the one he lost.

Hi, this could be condensed down a bit:

A former underground boxer suffering from a trauma induced brain injury is forced to partake in one last lucrative fight in order to pay for lifesaving surgery for his unborn child.

good luck

Hi, the above is quite long. The ideas not bad.

A condensed version. I just threw this together, it could be refined:

A former underground fighter suffering from a trauma induced brain injury must put his life at risk to partake in one last fight in order to make enough money to pay for life saving surgery for his unborn child.