Man, I was surprised when this roller cover didn’t fit the roller cage. I can’t say I’ve ever had that happen before! You cant just “use it anyway,” because the plastic tip of the cover holder will accumulate paint / primer but won’t hold it, so there is a great potential for paint to get slopped or dripped all over the floor or walls.
These are the 6/$7 disposable roller covers I get from Home Depot. Luckily, I had extras, and the second photo shows the proper size and fit.
I use them for rolling on wallpaper primer, essential for every wallpaper job.