You know looking at the previous battles at the league( I know Ash vs Kukui isn't quite a league match)which were the final battles that weren't 1vs1 where Ash wound up losing if you look at it Pikachu always gets a win or at least a draw in the final league match and here is how this breaks down:
Kanto league: Pikachu got a win but had to recalled from the battle against Ritchie's Charmander(Aka Zippo) which was treated the same as Pikachu losing(which in later battles this wouldn't be the case).
Johto league: Pikachu gets a 1 win against a Kecleon but then loses to Sneasel.
Hoenn league: Pikachu got 1 win and then lost to Meowth.
Sinnoh League: Pikachu tied with Latios.
Unova: Pikachu got 2 wins and lost to a Lucario.
Kalos: Pikachu got 2 wins and lost to Charizard.
So looking at it, it seems like they try to have Pikachu get a 1 win or a draw in the final major match that isn't a 1vs1. So now looking at the battle this means that if Tapu Koko is going to be Kukui's "sixth" pokemon instead of the one Kukui plans on using then that means unless Naganadel or Torracat lose to Lucario or Incineroar, that Pikachu will need to beat Tapu Koko or tie with it to avoid ending the match without getting a single win.
Which given how Braviary beat Lycanroc and Empoleon was able to forced Ash to recall Pikachu despite the initials predictions which were before we found out about Rowlet battling Braviary having been that those 2 would most likely lose to Lycanroc and Pikachu, then I don't think anyone should count out the possibility of Ash's Naganadel beating Kukui's Lucario.
I mean after all if you look at it so far when Ash and Kukui sends out a pokemon the pokemon with the type disadvantages winds up winning or it's opponent is recalled before they can knock it out, the only exceptions to this were Kukui's Incineroar being recalled against Ash's Dusk Lycanroc, Ash's Torracat beating Venusaur, and Kukui's Incineroar beating Ash's Melmetal.
However given that we know that Ash's Torracat and Kukui's Incineroar still have to settle their rivalry it's really not that surprising that they've been able to win against pokemon they have a type advantage in the current match. You know come to think of it if the remaining match breaks down like this:
Naganadel vs Lucario: Naganadel
Naganadel vs Incineroar: Incineroar
Torracat/Ash's Incineroar vs Incineroar: Torracat/Ash's Incineroar(if Torracat does wind up evolving)
Torracat/Ash's Incineroar vs Tapu Koko: Tapu Koko
Pikachu vs Tapu Koko: Pikachu
it would wrapped up pretty nicely considering beside it giving Naganadel a win, it would give Kukui's Incineroar 2 wins, and Ash's Torracat/Ash's Incineroar 2 wins as well and then it allow Tapu Koko to have a win, while Pikachu gets it's win for the battle by finally be able to defeat Tapu Koko.