Wait, I thought only one month had passed since the ending of the first game...

Anyway, I do agree there is a lack of proper justification.
In some ways, it almost seems like there was at some point of development an intention to use the torture he suffers on lavalete castle for this purpose.
Gameplay wise he could have been presented as fully leveled and powered up in the prologue fights (which would also have served as a more palatable introduction to game mechanics), and then reset to level one from the point of the escape from the cell. Narrative wise being presented as the result of brutal torture he only survived because of his enhanced physiology