Prisoner litigation is a specialty area of law. Because of the Prison Litigation Reform Act, most claims cannot be brought in Federal Court. The most common reason is failure to exhaust administrative remedies. If you did not file the proper grievances while you were in jail, a Federal (Civil Rights) lawsuit against the jail will likely fail.
Google PLRA for more information.
Mcarp would have to follow state and federal tort claim procedures in order to file a suit.
Although it is
possible that the County of Riverside could be held liable if it can be shown there was inadequate supervision or the prisoner housed with Mcarp was known to have violent tendencies (with negligence shown by officials in not anticipating possible violence against cell mates), fighting sovereign immunity is not something one can easily do on one's own. Mcarp would need to consult with an attorney in his area.
I still see that the most viable claim would be against his attacker, although it is possible his attacker has a good defense that we currently know nothing about.
From what I have read of Riverside, this is definitely not a place where you want to be jailed (so it is best to obey the law so you don't wind up there).