Career
Holzer was drafted in the fourth round, 111th overall, of the 2006 National Hockey League Entry Draft by the Toronto Maple Leafs. Drafted from playing in his native Germany with European Commission Bad Tölz, he made his debut in the top Deutsche Eishockey Liga (DEL) with the DEG Metro Stars. Holzer began the 2010-2011 season with the Maple Leafs" American Hockey League (AHL) affiliate, the Toronto Marlies.
On November 3, 2010, he was recalled from the minors to make his National Hockey League debut on November 6, 2010.
In the 2012-2013 season, Holzer was recalled by the Maple Leafs on February 5, 2013, and scored his first National Hockey League goal on just the fourth shot of his National Hockey League career against Michal Neuvirth of the Washington Capitals. In the 2014-2015 season, on October 6, 2014, the Maple Leafs waived Holzer.
Unclaimed, he was later returned to the Toronto Marlies before on November 15, the Maple Leafs recalled Holzer on an emergency, only to send him back down to the Marlies on November 16. One week later, Holzer was recalled and remained with the Leafs to replace injured defenceman Roman Polák.
On March 2, 2015, Holzer was traded by the re-tooling Maple Leafs to the Anaheim Ducks in exchange for defenceman Eric Brewer and a fifth-round pick in the 2016 National Hockey League Entry Draft.
Holzer has represented the German national team in the 2010 Winter Olympics and 2010 IIHF World Championship.