I still play a 917F 3W, but have not had any issues like that. However, it seems like a pretty easy fix, so long as the missing portion came out in one piece and you still have it. Just clean and sand out the channel and use a good adhesive that will stick well to the old material. A polyurethane marine caulk would likely work just fine. Something like the marine industry brand, BoatLife LifeCaulk. Silicone based sealants aren't necessarily that good at adhesion. At least, not those used in the marine industry.

Or, if you'd rather not take the chance, I'm sure you can send it back to Titleist. I bet they have just the right product to repair it. ;-)

Saw this on Facebook blows my mind

AYoung
I jJust checked my 917F2 and the rubber piece was still in. Not sure if you can get a replacment piece. You may have to fabricate one yourself and apoxy it in.

Deno

Titleist have been awesome as usual and will replace the head with a TSI3 fairway head for a solid discount.