2010-11: Philippe Desrosiers played for the College Antoine-Girouard Gaulois midget AAA team in Quebec and won a silver medal with Team Quebec in the 2011 Canada Winter Games. Sharing the goaltending with Philippe Cadorette, he was 10-9 with a 3.54 goals against and .885 save percentage. Desrosiers was 2-3 in the playoffs with a 2.45 goals against and .907 save percentage. Desrosiers played two games at the CWG and had a 3.71 goals against and .837 save percentage. He was selected by the Rimouski Oceanic in the second round (29th overall) of the 2011 QMJHL Entry Draft.
2011-12: Desrosiers made his QMJHL debut, playing in three games for Rimouski, and played midget AAA hockey for College Antoine-Girouard. He was 1-2 in his brief stint with the Oceanic and had a 3.48 goals against and .899 save percentage. In 23 games for College Antoine-Girouard he was 16-4 with a 2.77 goals against and .907 save percentage. The Gaulois reached the Quebec midget AAA semifinals and in 11 playoff games Desrosiers was 6-3 with a 2.58 goals against and .905 save percentage.
2012-13: Desrosiers played 43 games for Rimouski in his first QMJHL season and was in net for Canada's gold medal team at the 2013 U18 World Junior Championship. He was 22-8-5 with a 3.07 goals against and .900 save percentage for the Oceanic. Rimouski finished second in the East Division. Desrosiers played four games in the Oceanic's six game playoff series with Gatineau and was 2-2 with a 2.26 goals against and .892 save percentage. At the U18 WJC in Sochi he was 5-0, including a 33-save effort in the gold medal game win over the USA; finishing with a 0.80 goals against and .970 save percentage. Desrosiers was ranked seventh amongst North American goaltenders in Central Scouting's final rankings prior to the 2013 NHL Draft.
Aggressive, confident butterfly goaltender that has a surprising amount of athleticism and quickness. Can get out of his butterfly stance just as quickly as he can get into it. Anticipates the play well and challenges adeptly. Advanced rebound control at his age. Understands the concept of directing rebounds. Actively makes saves, doesn't just block shots. Moves pretty well in the crease, mainly because he doesn't give up his leverage to push across entirely because he's not a "lazy" butterfly goalie - he's an active player. High shots can be problematic for him. No young goalie is ready out of the box, still holes to fill in his game.