Yes you can injure your testicles permanantly by squeezing them too hard - why are you doing that in the first place - It can't be comfortable!
Massaging testicle is one of the technique to increase production of sperms. However, one has to massage the scrotum, putting less pressure on testicles.
So how much pressure can injure them?... And what are the symptoms that show that testicles are injured?.... Is there any swelling in testicles or inflammation ?