Complex sentence: He quickly became a regular in the team, making 55 appearances in the 1999--2000 season and 33 appearances in the 2000--01 season, although he was dropped from the side in February 2001, which prompted him to hand in a transfer request.
Split into simpler sentences: He quickly became a regular in the team, making 55 appearances in the 1999--2000 season and 33 appearances in the 2000--01 season. Although he was dropped from the side in February 2001, which prompted him to hand in a transfer request.