Again, power scaling in One Piece is NOT consistent
Oda wrote his story like that:
1- Inuarashi and Nekumamushi fighting Jack to a stand still (while tag teaming, so they ave ability to rest)
2- Asura delivering better blow to Jack in mere one chapter, and looking like a true equal to Jack
3- Inuarashi LOSES a leg. Legs are of GREAT IMPORTANCE for swordsmen balance! As a swordsman, he should have lost a big chunk of his strength.
4- He fights Asura equally after that
It doesn't make sense at all.