If age comes into it its a much tougher arguement, the skies the limit for Musiala as hes gotten better every season it seems. Our team without him just looks much more lethargic right now, he creates space all around him when hes on the ball.
All that being said there are more #10's around that could slot in, not to say easily replaceable but there are allot more options then say trying to replace Kane. It was the initial problem when Lewy left as well, Kane was the seemingly obvious fit but there really wasn't anyone else. I guess thats why we tried the Mane route and a formation change which clearly failed.