Cant really judge his accuracy or completed passes rate, but you can take out of the footage around two main features that makes you believe he will improve a lot our current midfield. First of them is that he doesnt take much time on the ball before releasing the pass, be it short or long, one or two touches at most and secondly when he does he can pass with both feet so that he's got a big range os positions from where to release a pass and eventually start and attack or recycle the ball. Thats something we are really lacking atm, and you could see it in the perth game, a lot of time was needed before a pass, be it forward or sideways, just see Matic, and if he can print some of that in our midfield I believe he will be an upgrade.
Also, it cannot hurt a lot to add numbers and depth to the squad, even with the tag of unproven, which, as someone pointed already, deasn't really translates that hes going to be shite.
On another hand, if Garner is able to step up and kickstart his carreer so that by the age of 21 he's better than Longstaff the who wouldn't rather that? give him a chance anyway.