What good he be if he missing through injury when we need him?Because
1) we already have a trusted right back who is young and also one of our fittest players so we don't need a big investment in the position.
2) Mazraoui kills two birds with one stone because he can cover left back as well possibly buy Amass a chance to make a claim for the position long-term.
3) he's relatively cheap and I doubt he'll be on big wages, so it wouldn't be so tough moving him on when he looses relevance and he can be really good for us in relation to his price tag.
4) he already knows the manager demands and have thrived under him.
Missed 78 days and 19 games due to various muscle injuries.
Missed 72 days the season before that.
Missed 62 days the season before that.
He could be free and play for us on minimum wage but he'll have to available to help our cause. Just being 'cheap' is not a justification for signing a player.