Of course Yamato can save Luffy, but don't you think it's a bit too late for that?
Yamato states that she has to hurry after she finds out that only Luffy is left on the roof - so the only way she will find out Luffy falls is to reach the roof - so we assume Oda will skip:
- reaching the roof
- some short conversation with her father to find out what happened,
- and most important thing: Luffy in Chapter 1013 was already a good piece under the island.
I think that Yamato will initiate a flasback on the roof - apart from the fact that even if Luffy managed to return to the roof immediately - it still does not change anything, he is still too weak now.