The players are fine (individually). That includes Sandro. Almost universally though, the job they were bought to do or the price paid for them (or both) is wrong. That is on the manager.
See:
In terms of how we play attacking passes, we've played the 4th most crosses-per-throughball of any team in the league. That is not a team for Sandro to play well in. He was not bought for any reason other than the fact that he's young, is a good player, and was insanely cheap.
Note that I have no qualms with that as a general approach to general talent-hoarding (from a club's point of view, though it's a shame for the player) - but it does mean we can't draw any conclusions on Sandro's quality. There is, however, the ugly spectre of Koeman genuinely believing that Sandro would play well in, and add goals to, his system. Which doesn't say much for his tactical acumen.