CV joint retaining ring gone

I took my jeep grand cherokee in for an oil change and the kid who did the job said he noticed the CV boot on the passenger side was torn. I thanked him and when I got home I took a look and actually the problem appears to be the retaining ring on the large end was gone. The boot itself looked to be in good shape. My question is this: do i need to repack the whole joint just to replace the retaining ring, or can I shoot a little grease into the joint and close it all back up?