May Arrest Not Substance Abuse Related

Ryan Leaf's domestic battery arrest was NOT substance abuse-related ... so says the ex-NFL QB's attorney, who tells TMZ Sports the former San Diego Charger is still "100% sober."

There had been some concern after 44-year-old Leaf -- who has battled drug and alcohol problems in the past -- had suffered a relapse that led to his arrest on May 22.

But, when we spoke to David Greenberg of the Law Offices of Soda & Greenberg ... he told us that just wasn't the case at all.

"He is 100% totally sober," Greenberg said of Leaf, "and spends a good part of every day over the last five years helping other people remain that way."

In fact, Greenberg says the whole alleged incident was nothing more than a…