Westermann started his men's club career in 2008, playing for Centre Fédéral de Basketball, in the Nationale Masculine 1, the third division of French basketball. He played for Centre Fed for two seasons, averaging 10.1 points, 2.6 assists, and 1.1 steals per game, in 48 games played.
On 2 July 2012, it was reported that Westermann had signed a three-year contract with Serbian club Partizan.[5] With Partizan was one of the candidates for the prestigious EuroLeague Rising Star award, which eventually went to Kostas Papanikolaou.[6] In his first season, he averaged 9.6 points, 2.6 rebounds, and 4.1 assists per game in the EuroLeague. In the ABA League season, Westermann helped his team to win the title, averaging 6.9 points, 2.5 rebounds, and 2.2 assists per game, in 28 games. In November's EuroLeague away game against CSKA Moscow, he tore his ACL on his knee, which sidelined him for the remainder of the season.[7][8] As a result of the injury, Partizan had to find a replacement for him in Milenko Tepić, a former member of the team.[9]
Barcelona / Limoges
On 17 July 2014, Spanish club FC Barcelona acquired the player rights of Westermann.[10] On 25 July 2014, he was loaned by Barcelona to the French EuroLeague team Limoges CSP, for one season.[11] Over 10 EuroLeague season games, he averaged 7.7 points, 2.3 rebounds, a career-high 4.3 assists, and 1.3 steal per game. On 11 July 2015, Westermann signed a one-year extension, with the option of another year, with Limoges.[12]
Žalgiris
On 30 May 2016, Westermann signed a one-year contract, with the possibility to extend it for one more year, with the Lithuanian club Žalgiris.[13] Over 30 EuroLeague season games, he averaged 8.2 points, 3.2 rebounds, 5.5 assists, and 0.5 steals per game.
CSKA Moscow
On 22 June 2017, the Russian club CSKA Moscow, announced the signing of Westermann, to a 1+1 (2nd year being optional) year contract.[14] On 16 July 2018, Westermann was officially released from CSKA.
Return to Žalgiris Kaunas
On 23 September 2018, Westermann returned to Žalgiris and signed a one-plus-one-year contract.[15]