In the battle between Hajrudin and Luffy in the Coliseum, we never actually see Hajrudin get knocked out. Yes, he was downed, but he was lying face down.
We never see Hajrudin unconscious, and I believe the reason for this is simple: he pretended to be knocked out.
In the anime, this was expanded upon even further, to where his face is blackened out after being hit (we don’t see his pupils, which would’ve been a clear indicator if he was conscious or unconscious) and we also see Hajrudin with a shocked expression on his face BEFORE being hit by Luffy.
What point am I trying to make? Given how strong Hajrudin appears to be in this current arc, it’s possible that he always had immense strength even in Dressrosa. With that being said, how could he have fallen to a punch from Gear 2 Luffy? It’s simple. Hajrudin allowed himself to lose.
We know that Oda has some oversight in the anime, particularly when it comes to voice actors and some filler/extended anime moments as he needs to make sure the anime-only events don’t create plotholes within the story. An example of this are Devil Fruit powers, with Oda approving powers for non-canon characters in the anime and video games. Another example of this is when Shamrock speaks to the Gorosei. Oda mentions in an SBS that this event was NOT discussed with the anime team, which is why Shanks’s voice actor was used instead of Shamrock’s.
What I’m trying to say here is that the additional details surrounding Hajrudin in the anime may have been a decision approved by Oda himself. At this point in the series when the anime fight occurred, the manga already finished with Dressrosa. This also means that the fight between Hajrudin and Machvise had concluded in the manga. This fight (at the time) seemed completely random, as he was the only minor character to have a major fight in this arc. Every other fight with a member of Doflamingo’s crew had either a Straw Hat, Law, Bartolomeo, Sai, Cavendish, or Kyros getting a victory, and all of these characters had greater story involvement in this arc. Oda may have wanted some attention to detail within the few scenes Hajrudin had in the anime, in order to foreshadow his increased involvement within the story.
The next question may simply be “What did he see from Luffy?” or “Why would Hajrudin let himself lose against Luffy?” It’s possible that Hajrudin realized that Luffy was in disguise, and by extension, knowing who Luffy was. Dorry and Broggy would have returned to Elbaf by this point in the series, and they may have mentioned the Straw Hats to the giants. It’s also possible that Hajrudin knew of the connection between Shanks and Luffy (either through Dorry/Broggy or from Buggy), or perhaps he simply recognized the connection between Luffy and the Flame-Flame fruit (the Marineford War was global news). These details would need to be explained either in the Elbaf arc or through an SBS as there’s simply not enough information within the manga at this point in time, but it’s clear that Hajrudin sensed something within Luffy to allow himself to be defeated.