[2] Accordingly, he joined the major junior Western Hockey League (WHL) with the Prince Albert Raiders.
Upon completing his first season in the WHL, Thelen was assigned by the Wild to their American Hockey League (AHL) affiliate, the Houston Aeros, where he made his professional debut, playing in one game.
The next season, in 2006–07, after missing some time due to a knee sprain, Thelen was traded by the Raiders to the Vancouver Giants, in exchange for defenseman John Flatters and prospect Nathan Deck,[2] where he captured the 2007 Memorial Cup championship with the Giants as tournament hosts.
Unsigned by the Wild out of major junior, Thelen turned pro in the ECHL with the Texas Wildcatters, tallying 35 points in his rookie season.
[3] He attempted to come back a week later, but ended up sitting out the rest of the season due to post-concussion syndrome.