Jon Jones was devastated when he got the call about a positive steroids test -- so says JJ's teammate, who claims there's NO WAY Jones cheated and blames the whole thing on a "setup."

Frank Lester -- who trains with Jon at Jackson Wink in New Mexico -- says, "I was just at dinner sitting next to Jon Jones when he got the phone call about this failed drug test."

"Supposedly the story is between weigh ins and fight night he took an oral steroid. Which makes zero sense since it wouldn't do anything at all for Jon to do so. It takes weeks for steroids to build up in your system to be a performance enhancer. Jon was devastated to say the least."

"This is a setup, straight-up, no athlete would test clean his entire fight camp, and then randomly take some cheap f*cking oral steroid between weigh ins and fight night knowing he would be tested once he got done fighting."

"This is a straight setup. They are trying to ruin this kid's life. It makes no sense and Jon wouldn't do it."

"Let's see what happens with all of this but I truly believe Jon is innocent & I hope that all of you will give him a fair chance before you send hateful judgments his way."