It is clearly not a zero risk, the early briefings were that we didn't want to get him in Jan to avoid upsetting squad dynamics.
Equally Pep who knows him has always wanted him, Txiki presumably knows him too, so they obviously consider the risk acceptable. The alternative scenario of getting no one cripples us with further injuries and could damage us just through over-playing.
There are risks in whatever we do, Sanchez didn't seem to damage Barca when he was there and improved Arsenal for most of his time there.
I think we'll be fine but accept that there is always a level of uncertainty.