At least one of Irving Fryar's old teammates still has his back -- ex-Miami Dolphins star Keith Jackson ... who says he believes Fryar is a good person at heart and is very concerned that he'll be spending the next 5 years in prison.
Jackson tells TMZ Sports ... he's remained friends with Fryar ever since they played together back in the '90s -- "That's my boy. I love him like a brother."
As we previously reported, Fryar was sentenced to 5 years behind bars this week for bilking people out of $1.2 MILLION in a mortgage fraud scheme.
"I'm concerned," Jackson says about the sentence ... "It's not good."
Jackson says he has a hard time accepting that his friend is now a convicted criminal ... but he's refusing to turn his back on Fryar.
"I'm still with him. I'm standing with him. It's hard for me to believe he did this... he was always a trustworthy, good person."