First I want to say that Sanji is definitely stronger than everybody thought, capable of blocking someone so strong like King.
In the latest chapter (988) Sanji blocked 2 of King’s attacks. The first block is shown in the following picture:
As you can see in the first circle (1) Sanji’s foot makes contact halfway between King’s knee and boot, and in the second circle (2) the sound effect akin to that from a clash of swords is employed, implying Sanji and King both used armament haki.
However, the exchange of legs continues into the second panel where Sanji blocks another of King’s kicks:
In the first circle (1) of the above picture you’ll notice that in this block Sanji has actually used his knee, causing black lightning visual effect (sign of armament haki). This is accompanied by a different sound effect highlighted in the second circle (2).
An important thing common to both pictures and clashes is that Sanji blocked without budging or being overpowered. This indicates that RS Sanji in base form is strong enough to clash evenly against base King which is really impressive.
However, King is definitely stronger than Sanji and markedly so.
In the above panel Sanji was quite clearly struggling to (unsuccessfully) free himself, indicating King out-classes Sanji in Full Zoan form.
And this great difference between the two is further accentuated when we see the impact of Full Zoan King’s attack when Sanji’s eyes strain from the pain:
And what’s worse is that Sanji was positioned to feel the full brute force of King’s deadly attack…
…which culminates into a scream from Sanji.
Even Luffy acknowledges the deadliness of the attack.
Obviously Sanji isn’t going to die, so there are 2 outcomes that I think are possible going forward:
- Sanji survives and is surprised at the durability of the suit and we get a hint of the full extent of the suit’s capabilities.
- Sanji reels from a devastating blow, but isn’t quite dead causing King to continue following through on his commitment to take Sanji out of the picture.
If “scenario 1” happens then Sanji will be in similar position to Luffy when he was fighting against Katakuri. He’d be out-classed for an extended period of time, but will still stay in the picture where eventually he can unlock a stronger haki in the heat of battle and win.
Luffy also was beaten and out-classed hard by Katakuri at first and he even asked Katakuri to stop his attack, but in the heat of battle he grew stronger and unlocked better observation which helped him pull a difficult win.
I think the same is going to happen with Sanji.
*Theory by AllBlueReverie