I don't think the position has changed all season to be fair. We have been a Pelle injury / suspension away from disaster in this area.
I don't personally think the injury to Long is a change here, we've only played him as lead striker in a couple of games and he has otherwise been deployed as wide in the front three or a second striker. In those two wide positions we have Tadic, Elia, Mane to rotate. If we play with two up top Ron has deployed Mane, Elia and Long there, to good effect.
What Long does give us is flexibility in formation and an option from the bench.