I lived in Ohio at the time. They had the logs from his bookie. Never bet against the Reds based on the evidence presented by the prosecutor.
He didn’t need to bet against the Reds to give himself an advantage. If you’re the manager and you’re betting on your team to win on some days and other days you’re not, it’s certainly going to influence your managerial decisions in how you use the resources available to you. Especially when you’re a degenerate gambler and an all-round asshole generally.