Terrible news this injury. Hope he's back soon and stay healthy!
Indeed a bad news. I do hope he'll be fine very soon.
He is a great talent and already the sole character to worth watching - besides Messi.
But as a matter of fact, Dembele is only "needed" for this crucial game because the other players (apart from Messi) offer less assurances for an easy win.
A "normal" Barca
should easily win at home against Lyon, which is undoubtedly not in par with Barca, but people have their doubts here and elsewhere, just because this present team is not able to make you comfortably sit in front of the screen and assure you that they are always superior.
(Do not count very lucky wins please, like the 3:0 against Madrid, because no doubt Barca was the inferior and lucky extremely team there...)
If Messi picks a good day, success will come, but he is the only one you can bet on. The rest of the players are either unpredictable - if they'll have a brilliant, good, bad or horrible game - so things depend on disposition and luck.
If the worse screen-play gets into the movie, the team would indeed need Dembele's speed, creativity and unpredictability, but under "normal" circumstances he is not "needed" for the desired win.
Messi will be there, Suarez can still score, besides his present lack of ability to handle a simple pass or ball, Coutinho's curse may be temporarily lifted by the gods, Malcom could score a lucky one as he has done that in the CL series, Pique's head could be vital and Alba's runs on the left could be successful once again, even if all teams know by now the pass he will make back to Messi... etc.
But the important scene to see is the change of good-souls towards Dembele.
Little over two month ago the folks were furious with him because he was late once or twice (as if it was an important thing compared to his performances), wanted an immediate sell and the like, so it is fun to observe that by now he is the "needed", missed, liked and crucial ingredient for the win...
Times seem to change...