Don't glue it, at least not unless you grease it regularly. That part involves quite a bit of pressure against the other, and you'll wear it down if you do. I'm sure that's the reason for the roller at all. Before you lose yours, try and find a washer or clip that will hold it in place. There is clearly a groove on the bottom which is meant to hold it there.
I plan to find one online or at a hardware store, but I didn't know the dimensions. I'll probably get around to measuring the replacement part and figuring it out from there. It would be great if someone already knew. It would save quite a bit of time.