Irving Fryar -- Star NFL Teammate Concerned Over Prison Sentence

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.