Complex sentence: The 2013--14 season was a breakthrough for him, as he established himself in the first team and later in late January, Buchtmann signed a new contract with the club, which will keep him until 2016.
Split into simpler sentences: The 2013--14 season was a breakthrough for him, as he established himself in the first team. In late January, he signed a new contract with the club, which would keep him until 2016.