But the thing is during the flashback of Rebecca and Soldier-san/Kyros, it does appear that Soldier-san/Kyros had both legs when helping Scarlett and Rebecca escape Diamante and his men:
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In the next page of that chapter we see that Soldier-san/Kyros has only one leg - his right leg, and his body is quite beaten up:
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If Soldier-san/Kyros did have two legs while holding off Diamante and his men while Scarlett and Rebecca escape, then he either lost his left leg during that scene or when he was trying to protect Scarlett when she went to get food for Rebecca.
I'm leaning more towards Soldier-san/Kyros losing his left leg to Diamante as Diamante utilises a sword and can extend it/make it flutter with his Hira Hira no Mi (makes his attacks somewhat unpredictable). Diamante could also be the reason for why Soldier-san is so beaten up and why his hat is dented - a victim of Diamante's Corrida Glaive attack? =/.
This could explain why Soldier-san/Kyros was unable to protect Scarlett, as he just had his left leg cut off recently and was not accustomed to using only one leg, which made it harder to protect Scarlett.
With Diamante likely being the culprit behind Rebecca's father's lost leg and her mother's death, the match-up between Diamante and Rebecca seems like quite the deliberate one by Oda. Given the nature of this arena round and how it supports Rebecca's fighting style, I can see how she can go about combating Diamante, especially if Bartolomeo uses his barrier in a certain way in conjunction with a tactic I believe Rebecca can effectively use.
I believe the wound Soldier-san/Kyros received during his 3,000 wins in the Corrida Colosseum is unrelated to the Donquixote Pirates and is connected to a separate story.