Injury substitutions

How do injury subs work? I've just had a game where my number 8 got injured. While I didn't have another healthy 8 on the bench I did have a 6-7 and a 4-5. Instead I had my lowest rank no 15 auto subbed in. I wasn't able to watch the game live so couldn't correct it.

My lowly 15 was at the bottom of the list on the strategy page which I though would have rendered him last in line to join the game.
Normally it would take the best substitute with the nearest post. I will have a look, maybe there is a bug.
Just had another game where my number 3 was injured. He was replaced by a number 10 despite there being 2 higher ranked number 2s and a 4-5 on the bench. I do happen to have a number 8 playing at 3 but that shouldn't cause a 10 to be the designated sub.
That's because you has no 1-3 available in your substitutes
But surely it should pick a 2 4-5 6-7 or 8 rather than a back :o)
Exactly Taff.
I have this problem regularly. Sub seems to be picked at random.