That's not a thing down here in England.
There are a couple of different types of manslaughter which relate to unlawful or dangerous acts, so it'll all obviously come down to why and what he thought he was doing with his leg in the air, and whether that was dangerous/whether he was acting unlawfully.
By giving him a lot of time since the incident, there will no doubt be a lot of evidence for the Police and CPS to comb through (messages to family and friends, team mates etc) which is probably why he's being held for so long, and probably why he was arrested rather than just interviewed on a voluntary basis (so they could seize electronic devices).
If he's even hinted to somebody that his movement (not the incident) may have been intentional, he's struggling; it's quite clear that players know how dangerous the blades are.
I have sympathy with him for the outcome and for any consequences that follow as I'm not sure he could possibly have imagined in a split second what would happen, but from watching the video I think he's going to be in big trouble here unless he can explain that movement fully with corroborating evidence.