2008-09: Sebastian Wannstrom spent most of the season with Brynas at the junior (J18) level. In 32 games, Wannstrom scored 11 goals and added 9 assists for 20 points. Wannstrom went unselected in the 2009 NHL draft.
2009-10: Wannstrom had the most goals (30) and most points (57) in the J20 SuperElit, in 35 games for Brynas. In the SEL, he was held scoreless in 18 games.
2010-11: Wannstrom saw limited ice time as a 19-year-old in his first full season in Sweden's Elitserien. He averaged under five minutes of ice time per game and was -4 with 2 assists and 6 PMs in 45 games for Brynas, which finished seventh in the 12-team league. He skated in five playoff games and was +1 with no points or penalties. Wannstrom competed for Sweden at the 2011 U20 World Junior Championship. He scored 2 goals with 3 assists and was +1 in six games as Sweden finished fourth in the tournament. He signed a three-year, entry-level contract with St. Louis in June 2011.
2011-12: Wannstrom signed a three-year entry-level contract with the Blues in June 2011 and returned to Sweden to play in the Elisterien on loan to Brynas. Skating in a lower line role for Brynas he scored 8 goals with 7 assists in 43 games and was minus-five with 20 penalty minutes; averaging 7 minutes of ice-time per game. Brynas won the league playoff championship after finishing fourth in the regular season. Wannstrom was plus-seven with 2 goals and 5 assists while skating in all 17 playoff games.
2012-13: Wannstrom struggled to find ice time in his first professional season in North America, and was also slowed by a case of food poisoning: before returning to Sweden to finish the season in the SHL with Brynas. With a deep lineup due to the NHL lockout, Blues AHL affiliate Peoria offered little opportunity for the 21-year-old. Wannstrom scored 1 goal with 2 assists and was -6 with 4 penalty minutes in 16 games before being sent to ECHL Evansville in November. He scored 6 goals with 1 assist and was +2 with 12 penalty minutes in 14 games for the Icemen. Wannstrom returned to Brynas in February rather than being a healthy scratch with Peoria. In nine regular season games he was -5 with no points and 8 penalty minutes; averaging 12 minutes of ice time. Brynas captured the final playoff spot – finishing eighth – and Wannstrom was -4 with no points and 2 penalty minutes in the four-game series with first-place Skelleftea.
Wännström is a strong skater that creates a lot of space with his speed for him and his linemates. The skilled player has offensive upside potential but has enough of an all-round game to be used on a scoring line as well as on a checking line. The Swedish-born forward will need to adjust to the speed and style of the North American game.
Wannstrom came over from Europe to try his luck in North America during the 2012-13 season, but he never found his stride in the AHL or ECHL. He will likely have another chance to show his skills at the pro level in 2013-14.