I would guess that it’s not going to be that easy to get a reliable big man. Dwight Howard is still free agent as far as I know and I do not understand why he is not picked up, for that case Melo is still free agent as well. Instead of forcing them to early retirement, they could just hire them and put them in the bench and even if they do not play any meaningful minutes, they could be getting like 5 minutes per game to keep fresh and always be there, which would give them a good training as well.
If any player gets injured badly, they would have a backup plan with these two, and they would come out and play 15-20 minutes instead of 5 minutes in case of an injury.
Yes, that's a fact and I can hardly imagine that the Lakers won't get another player of the same level as AD to help with the rotation.
I even imagine this is one of the critical problems of the Lakers, finding a big man player with similar AD level to replace him during some games.
Dwight Howard after leaving the Magic, practically his career plummeted and we never saw him play like before and Melo is also at the end of his career and both are free agents.
I imagine the Lakers will need to have other options in order to get another player to help AD when he's not very healthy.