Utah Jazz center Jeff Withey will NOT be prosecuted for domestic violence -- despite the fact cops believe there WAS a physical altercation with his Playboy Bunny ex-fiancee.
TMZ Sports has learned ... Withey was investigated by the Manhattan Beach PD after Kennedy Summers filed a police report accusing him of beating her up.
Investigators found evidence that Withey and Summers got into a physical altercation during an argument in July 2016 ... with Withey admitting he used force against Summers, which left bruises on her thigh.