If losing the ball carelessly was a reason for not getting picked, Rooney wouldn't be making the starting XI anytime soon again. If Barkley had given the ball away as much as him during that Man United game, it would've been held up as an example of why Koeman doesn't pick him.
That said, I think Lookman is better off the bench right now.
It isnt just losing the ball, it is where and how you lose it. Every time Lookman has played he looks like he has a lot of talent but not an understanding of how to use his talents fully and not a great appreciation of what is needed in what situations. For instance times when he has been in our half and he has tried to take two people on where there is minimal benefit for it working against passing it and moving forward into space whereas the risk if you lose it is huge. Or even in the other half when he has got past someone had space to drive into and he's slowed it down and come back.
I'd have him on the bench because if you need something to happen he is different to what we have, he can do something out of nothing and I really dont see much point in Mirallas any more aside from he plays with more sense than Lookman if you are winning. But I dont see a player who is ready for regular starts for us like a lot seem to think, I think its a case of people liking the idea of what he could be than the reality of the level he is at right now and people might get frustrated with him if he played 10 games in a row for losing the ball a lot, making daft decisions and frustrating more than he does good.
Not that there is anything wrong with that because he is a teenager and someone who came into academy football relatively late so he will have a different learning curve. We can all see the talent is there but same with DCL there is a clamour for him because of the problems we have in the squad rather than him being ready to play week in week out in my opinion.