IF the "rumors" mean 100% of pre-injury abilities, then we DO know what that looks like and it's REALLY good. A 100% pre-injury Jarrett Grace would be impossible to keep off the field. In that scenario, he and Jaylon would take two LB spots and Joe and Nyles would fight for the other.
IF the rumors mean "Jarrett has fully recovered as a normal human being", then he spends time spelling Joe at middle linebacker and reserve signal caller, while both Joe and Nyles likely get the reps with Jaylon.
Neither one one these scenarios is a bad one to imagine. ... especially if Nyles keeps improving his instincts for reacting to the correct gaps, and maybe figures out his pass drops better.
Only if the rumors are mainly pollyanna-ish baloney do we not improve significantly as a team. Jarrett Grace is a football player.