I don't really blame Sandro for the slow start because I have no idea what Koeman wants him do or if Koeman even knows what he wants him to do.
His credentials as a goalscorer were always a bit overrated but I still think there's a good footballer in there. The pass to set up Calvert-Lewin in the League Cup gave us a glimpse of what he can do given willing runners. We'd probably get better performances from him if we put him in lineups with better off-the-ball options (i.e. where Rooney isn't the only attacking player he has to pass to).
He's only 22, anyhow. Way too early to write him off, even though he hasn't been the can't-miss player a lot of people expected.