News of the engagement had barely settled down before Canseco leveled the claim on social media, an accusation he's been making against Rodriguez for years.
"Watching World of Dance watching J.Lo text Alex Rodriguez little does she know that he is cheating on her with my ex-wife Jessica poor girl she has no idea who he really is," Canseco began.
Canseco claimed to be around Jessica when A-Rod called her.
He added, "Alex Rodriguez stop being a piece of shit stop cheating on Jennifer Lopez."
The disgraced former MLB star then decided to stand up for J.Lo's honor and challenged A-Rod to a fight, writing, "Alex Rodriguez I challenge you to a boxing match or an MMA match anytime you want."
Canseco also said that he was "willing to take a polygraph to prove that what I'm saying about Alex Rodriguez is 100% accurate."
Alex Rodriguez proposed to Jennifer Lopez over the weekend during a romantic getaway in the Bahamas.