TBR i think last year he had 3 hamstring strains, his other substitution was getting knocked out.For what it's worth when you play Dr from afar, I'm firmly in the camp of Prestia having completely resolvable issues.
The thing is there aren't many non-modifiable risk factors for hamstrings and the absolute leader of that pack is previous injuries.
If he has a truly non-modifiable factor causing hamstrings like a hip deformity then there's no way he get this far as a professional athlete. Yes, you can get some referred pain from back conditions but these are all highly treatable.
In terms of modifiable factors it's things like hamstring weakness, fatigue, flexibility, and the balance in strength between hamstrings and quads. Then you have things like the strength and coordination of the core.
Your non-modifiable factors are age and previous injuries. Age is a non-issue with Prestia but previous injuries certainly are. If you have a hamstring injury you are much more likely to have another one and so on. That's partly because of the scar tissue that forms each time you damage the muscle but also ecause it is the most reliable indicator of the presence of the non-modifiable factors.
I'll bet every cent I have that Prestia's issue are because of a modifiable factor(s), which is being exacerbated by the non modifiable factor of the previous injuries. So it's a vicious cycle, he has a factor causing him to ping one, which lays down scar tissue and makes the muscle deal with the same factor but with a little less resilience. Ping again and on it goes.
The team around him will be analysing every factor and working to find the right mix in all areas but it is a slow and painful process and there is no magic formula (including calves blood injections which are complete *smile* in my eyes and the eyes of anyone with any regard for proper process of the practice of medicine) so can be one step forward, five back for a long time but I'm sure they will find the right mix at some point.
Is the fact that each one was a strain rather than a serious injury significant in any way, or would you think he is so aware of his hamstring he stops playing before he really pings it?