I don't know. Did Luffy took Arlong that serious. As far as I remember, I think Luffy was a bit careless and not taking the fight all that serious.
When he fought Zoro, he went serious from the beginning (look at his facial expressions), like he could not be careless with him.
Sanji was that much weakened against Arlong? I don't remember him being that injuried and even so he could not do jack against Arlong. If Zoro is on Sanji's level, that could imply Zoro still could not beat Arlong, but I don't know. Look at how Luffy acted later in his fight against Zoro.