Think about the physics of the spinning cutter head. That black piece is what keeps the lube within the balls where it belongs. Any deflection/vibration in one end due to failure causes deflection in the other end, which leads to that bearing's failure also.
Change both bearings. What is your time worth, cost of ruined wood, catatrophic failure which could lead to the cutter hitting the jointers housing? I have seen sudden failure of this class of bearings where the balls get loose, fly across the room, and clearances change by a 1/4 inch! This is not the time for false economy. Just the physical strain of removing and re-installing this bearing could lead to its failure. Be sure in installing that you press ONLY on the part of the bearing under the strain. Don't press on the outer race when pushing the bearing onto the cutter shaft. Use appropiate sized deep-well sockets to push the bearings on. Pipe nipples work also. The interference of steel to steel works (bearing race to shaft), the same interference transferred thru the balls cause microscopic scoring which later leads to failure and NOISE. When you removed the bearing from the cutter head: Did you nick the black piece with a screw driver? What unseen damage did you do? During forty years of industrial maintence, we always trashed any used bearing that had been pressed onto a shaft! What is the RPM of the head? Do you need to purchase a better rated grade of bearing? A couple of extra dollars /bearing can bring much additional bearing life.
