The 19-year-old Under-20 Argentina international, who also has the option to extend his deal by 12 months, has been tipped by manager Felix Magath to become a future Premier League star.
"He is a young player, a great talent," Magath told the club's official website. "He is, in my opinion, a player who can develop to play at Premier League and Champions League level."
Casasola added: "It's a great club and I'm very happy to be here. I know that the club has great fans and I also know that it has a long history."
The Cottagers, who were relegated to the Championship last season, have lost all three of their games in the league so far.