To be fair we could bring in another left winger, then use Mane as cover for Firmino when needed. He did quite wel when deployed in the striker role this season.
I agree, the front 3 rotate allot when they play together, that's what makes them so hard to defend against, Mane has really come on, he can finish and can hold the ball from defenders, he's so strong, if we bring someone in who can play on the wings, they can give that cover.
They will not be sat on the bench all season, there are plenty of games, if we had a player like that, we may get further in the FA/League Cup!
Then we have Origi if we want to change tactics and go more direct/run the channels, that extra quality might get us the goal that turns a draw into a win next season, we are talking fine lines here.