Help - Urgent - rear drive case splined shaft O ring

[Troll]

I'd replace it if I were you. That's where it goes, but if it got out of its groove, then it's damaged

[Bucko]

I'm in the middle of a tire change and decided to pop the rear gear case off the swing-arm to grease the input shaft splines.  I noticed when I pulled it off that there was a O ring floating on the shaft between the back of the gear case and the splined part of the saft.  There looks like a pretty big U shaped grove just behind the splined part - is that where the O ring is supposed to go?  Is is supposed to seal the grease into the 'socket' that the splined part of the rear case fits into?  I believe the O ring is  92055c on the fiche: