It's an offside so marginal that it's completely unreasonable to expect a linesman to spot it.
If you're complaining about that, you're complaining just to complain.
Also, I'm pretty sure Lovren is holding him. I'm pretty sure holding in the box is a penalty, even if the player being held is offside. So if you really want to get technical about it, a pen should have been called long before van Dijk booted Lamela.
Of course, calling a pen for that would have been dumb, just like your (and others) whinging about Lamela being an inch offside.