Santos has played between 39 and 48 games in 4 consecutive seasons. So he's not injury prone. Having said that he did miss a fair chunk of the season just finished, with a pelvic injury, although still played 32 times. We will need to make sure he's fully recovered.
As a backup this seems a sensible option, especially given he's a free signing and will command a low wage. Made League One team of the year 3 years in a row, so must be top league one quality at least. As a squad filler, in a position where we have only one senior centre back, if Darling leaves, seems like good contingency planning.