2008-09: Grenier missed much of the season due to injury – skating in two games for the Saint-Jerome Panthers in the Quebec AAA Junior Hockey League.
2009-10: Grenier was the fourth-leading scorer for the Saint-Jerome Panthers in his first full season with the club. In 51 games he scored 26 goals with 28 assists and 63 PMs. The Panthers finished fourth in the 14-team league. In seven playoff games Grenier scored 1 goal with 3 assists and 2 PMs.
2010-11: Grenier joined the QMJHL's Quebec Remparts in mid-season after starting the season with the Saint-Jerome Panthers. In 31 regular season contests as a 19-year-old rookie with the Remparts he scored 9 goals with 15 assists and was -1 with 6 PMs. After finishing first in the East during the regular season Quebec reached the QMJHL semifinals. Grenier was one of four Remparts to score at a point-per-game pace in the playoffs, finishing with 8 goals and 8 assists in 15 playoff games, and was +4 with 4 PMs.
2011-12: Grenier attended camp with the Canucks before returning to the QMJHL for an overage season. Obtained by Halifax from Quebec over the summer, he was the second leading scorer for the Mooseheads with 25 goals and 39 assists in 64 regular season games and was minus-two with 42 penalty minutes. Ten of his 25 goals were scored on the power play. After finishing second to league champion Saint John in the Maritimes Division, Halifax reached the playoff semifinals. Grenier scored 4 goals with 12 assists in 17 playoff games and was minus-two with 19 penalty minutes. He was tendered a contract offer by the Canucks in June 2012 before signing to play with EC Red Bull in Austria.
A late bloomer, Grenier pairs size, skating ability and offensive skill into an attractive package. Not breaking into the QMJHL until he was 19-years-old, Grenier is surprisingly mobile for his size and only scratching the surface of his potential. He has the vision and offensively ability to either set up a goal or score one himself, but he will be more effective as he continues to grow into his frame, add strength and use his body more.