Well, food would be the last thing on its mind afterwards, regardless of where it was bitten. Infection is the most common and deadly outcome of a fight between large predators, and it's essential they spend a lengthy period resting to give their injuries a chance to heal over. If spiny goes swimming, running and battering down fortified fences with open wounds and broken bones, it'll be dead within days.
The idea of the big showdown at the end of the movie makes much more sense, as it can provide a reason for the spino to cross into rex territory (if pursuing the humans thoughout the film, for whatever reason...), and takes into account that the winner would be out of commission for the rest of the film. Also ties back to the first film's predator showdown finale, and is probably a neater thing to build up to rather than open with.
