Reggie Bush is such a huge fan of USC quarterback Caleb Williams ... he tells TMZ Sports he thinks there's absolutely no ceiling on what the signal-caller can do with a football in his hands.
"He's a great young kid," Bush told us out at Dash Radio in Los Angeles this week. "The sky is the limit for him."
Of course, Bush would know a thing or two about the topic ... he balled out at SC from 2003 to 2005 -- and then went on to have a solid 11-year NFL career.