Armstrong was selected by the Houston Rockets with the 23rd overall pick of the 2001 NBA draft.[1] He was traded along with fellow Rockets draft picks Richard Jefferson and Jason Collins to the New Jersey Nets in exchange for the draft rights to New Jersey's Eddie Griffin.[2] In three seasons with the Nets, Armstrong averaged 2.2 points per game.[3] He only played 17 games in 2002–03 due to suffering from regular back pain. He was subsequently not on the 2003 playoff roster.[4]
Armstrong became a free agent in 2004 and signed with the Golden State Warriors, but was waived prior to the start of the 2004–05 NBA season.[5] Armstrong's final NBA game was played in Game 7 of the Eastern Conference Semi-Finals on May 20, 2004. In that game, the Nets would drop the series to the Detroit Pistons by losing the game 69 - 90. Armstrong played for 4 minutes and the only stat he recorded was 1 assist.
For the 2005–06 season, Armstrong moved to Italy to play for Roseto Basket. He left the team in November 2005 after appearing in seven games.[6] In February 2006, he moved to Spain and played 10 games for Calpe Aguas.[6]
On November 2, 2006, Armstorng was selected by the Dakota Wizards in the third round of the NBA Development League draft.[7] He was waived by the Wizards on January 20, 2007.[8] He was acquired by the Bakersfield Jam on February 8 but then waived on February 28.[8] He was then acquired by the Anaheim Arsenal on March 15 but then waived again on March 26.[8]
For the 2007–08 season, Armstrong moved to Poland to play for Kotwica Kołobrzeg. In 26 games, he averaged 11.2 points per game.[6]
For the 2008–09 season, Armstrong moved to Ukraine to play for Budivelnyk. His last game for Budivelnyk was on January 9, 2009.[6]
In February 2009, Armstrong trialed with Marinos de Anzoátegui of the Venezuelan Liga Profesional de Baloncesto. He was fighting for a position on the team with a fellow American Michael Gale,[9] but he ultimately did not make the team and did not debut for Marinos.[6]
