Derek Chauvin's loved ones remain in the dark over what exactly led to him getting stabbed in prison as well as his current condition -- and they're furious over it ... so says his lawyer.

Attorney Gregory M. Erickson -- who reps Chauvin -- tells TMZ the guy's immediate family still knows absolutely nothing about Chauvin's stabbing ... including how it happened and who did it.

Erickson says he spoke to one of Chauvin's relatives Monday ... and tells us a prison official contacted Chauvin's father to confirm he was in stable condition -- but no further details were provided whatsoever, which is frustrating the family.

Obviously, the media has been all over the story ... and the family has had to get their updates from news reports, which Erickson says is unjust and cruel.

He tells us he points the finger directly at the federal facility in Tucson, AZ -- where Chauvin is currently being held -- for being slow to the draw on updating his next of kin ... and for a lack of transparency in general.

Not just this ... but Erickson says the prison should've prevented this stabbing from happening in the first place -- noting Chauvin is the most high-profile person in custody since, arguably, O.J. Simpson ... and that any other inmate getting near him should've been kept at bay -- if not completely frisked to make sure weapons stayed far away.

Since the stabbing Friday ... Gregory adds he's attempted to contact prison officials 8 times, to no avail. As far as next steps -- Erickson says he and the family will weigh their legal options if they don't get more answers soon ... but it's unclear what those may be.

A Bureau of Prisons rep refused to discuss Chauvin's situation for privacy/safety/security reasons. But, they noted to us ... "we can inform you generally that the Federal Bureau of Prisons takes seriously our duty to protect the individuals entrusted in our custody, as well as maintain the safety of correctional employees and the community."

Chauvin is serving a lengthy sentence of more than 22 years behind bars after being convicted of murdering George Floyd.