Oakland Raiders head coach Jack Del Rio has Sean Smith's back -- sorta. 

Del Rio was asked about Smith's arrest over an alleged July 4 beatdown of his sister's boyfriend and produced a possible defense for the cornerback.

"He was defending his sister," Del Rio told SFGate.com ... "Obviously, things occurred that were taken issue with, so he’s having to defend himself right now.”

TMZ Sports broke the story ... Smith -- who signed a $40 MILLION contract last year -- was arrested for felony battery in Pasadena after cops said he beat and stomped the man so badly he was hospitalized in critical condition.

Smith's lawyer says his client is innocent and will fight the charges -- but Del Rio's comments essentially confirm he did get physical with the other man. So, the "It Wasn't Me" defense is out the window.

But even if Smith was "defending his sister" -- the law only affords someone the right to use "reasonable force" to stop an imminent attack.

Based on the injuries the man suffered -- multiple broken bones, unconscious when police arrived, critical condition -- it sounds like officials believe Smith exceeded that standard.

Smith's due back in court next month.