I prefer to look at unfortunate incidents and negative news in a positive light as much as that is possible.
Arthur's 3-4 week long injury is obviously going to impact the team negatively as he was our most in-form midfielder (by far) but the injury could have come at a worse time. We have a somewhat solid 6+ point lead in the league. We should be able to defeat Lyon without much trouble over 2 games in the CL without Arthur. Even though Arthur is expected to return for the return leg of that CL tie. We should also be able to at least score 1 goal at the Santiago Bernabéu, draw or win without him.
Hopefully this injury will give Aleñá more playing time and give him a breakthrough. Vidal too will be able to start in more games and grow as a player for us. Coutinho, even though I have little hope about him turning his situation around anytime soon, might step up in the midfield alongside Busquets and Rakitic/Vidal.
I am not really worried about the CL tie against Lyon. Sure, they are a decent team and should not be underestimated but if Arthur will be the difference maker between us going through or going out of the CL, we need to start from scratch as a club. My main worry is the league and more precisely Sunday's game away at San Mamés.