Jaguars legend Marcedes Lewis says it'd be a mistake for his ex-team to trade Leonard Fournette ... telling TMZ Sports, "They should keep him."
Leonard caused some huge headaches in Jacksonville this season ... the superstar RB was often injured ... he was suspended for fighting ... and he sat on the bench, ignoring his teammates in the final game of the year.
In fact, things got SO sour with the star RB -- team exec Tom Coughlin publicly ripped the guy and reportedly voided all the guaranteed money remaining on his rookie deal.
All of it has led to wild trade speculation ... but Marcedes says his former team should NOT make the deal despite all the drama -- saying Leonard's just too good.
"The way they're structured, and the way they do their things, it's always about running the ball first and I think Fournette is an amazing running back."
As for Blake Bortles ... Marcedes thinks a change of scenery might be good for that guy -- telling us, "That's a different situation."
There's more ... Marcedes also tells us where he thinks Antonio Brown will play football next year -- and gives us an update on who he hopes to close out his career with.