2010-11: Robert Hagg played for Tierps HK men’s team in lower Swedish hockey as a 15-year-old, skating in 30 games for the East coast club, and played for MODO’s U18 and U16 teams. He scored 2 goals with 9 assists and had 30 penalty minutes for Tierps HK. He was scoreless with 2 penalty minutes in two U18 games for MODO and scored 2 goals with 13 assists and 26 penalty minutes in eight games for MODO U16. Playing for Uppland in the TV-Pucken tournament for Swedish high school players Hagg led all defensemen with 4 goals with 5 assists in eight games.
2011-12: Hagg moved to MODO for the full season, spending most of the year with the U20 squad, and represented Sweden in the 2011 U19 World Junior A Challenge and 2012 U17 World Hockey Challenge tournaments in Canada. In 52 games for MODO U20, including eight playoff games, he scored 5 goals with 14 assists and was plus-11 with 48 penalty minutes. He played six games for MODO U18; scoring 1 goal with 4 assists and was minus-one. Hagg was scoreless with 4 penalty minutes at the WJAC and scored 2 goals with 4 assists and had 16 penalty minutes in six games at the WHC.
2012-13: Hagg split the season between MODO’s Elitserien and U20 Super Elit league teams – playing in 27 games with the men’s team and being named the top Super Elit defenseman. He played for Sweden in the 2013 U20 and U18 World Junior Championship tournaments, serving as an assistant captain on the U18 squad. Hagg had 1 assist and was minus-three, averaging 7:46 minutes of ice time, in Elitserien regular season play. Seventh place MODO played Farjestad in the playoffs but Hagg did not see any ice time in the five-game series. Hagg scored 11 goals with 13 assists in 28 games with MODO’s U20 team during the regular season and played for both the MODO U20 and U18 teams in the playoffs (both of which won silver medals). He also won a silver medal with Sweden at the U20 WJC scoring 1 goal with 1 assist and was minus-one in five games. Sweden’s U18 team reached the quarterfinals and in five games Hagg scored 1 goal with 3 assists and was plus-one with 12 penalty minutes. Hagg was ranked eighth amongst European skaters in the Central Scouitng final rankings and was selected by Philadelphia in the second round (41st overall) in the 2013 NHL Draft.
2013-14: Hagg signed a three-year entry-level contract with the Flyers in March 2013 after skating for MODO in the SHL; making his AHL debut with Adirondack at the end of the year. He skated for the Sweden U20 team in the 2014 World Juniors. Hagg scored 1 goal with 3 assists and was plus-1 with 10 penalty minutes in ten games with the Phantoms. Adirondack missed the playoffs; finishing fourth in the Northeast Division. Hagg scored 1 goal with 5 assists in 50 games for MODO and had an even plus/minus with 49 penalty minutes; averaging 13 minutes of ice time. In the two game play-in series against Linkopings he was minus-1 with no points and 4 penalty minutes. Hagg scored 1 goal and was plus-4 with 12 penalty minutes in seven games at the WJC. Sweden won a silver medal; falling to Finland, 3-2 in overtime in the championship game.
Hagg is generally sound in most aspects of the game, but doesn’t yet seem to have any dominant traits nor has been able to be consistent in his effort. He keeps things simple, but can sometimes miss assignments in his own zone and won’t be found making breathtaking rushes with the puck on too many occasions. Hagg can play the point on the PP, but doesn’t stick out as an offensive dynamo or as a dangler of sorts. He makes a sound first breakout pass. Hagg has a bit of a choppy stride moving forward. Knees are usually bent while his shoulders are parallel to the ice and his back is extremely straight, all combining to give him a stiff appearance. He has a great frame, is not shy of the rough stuff, and looks like he can be molded into a punishing defender. He doesn’t back down from challenges.
Hagg attended training camp with the Flyers and is with AHL affiliate Lehigh Valley in 2014-15 — one of the league's youngest players after opening the season with the Phantoms as a 19-year-old. His size and physical attributes suggest he will be ready for the NHL level sooner rather than later. Is still adapting to the positional aspects of defending but contributes to the attack. for the Phantoms.