Thomasson hasn't gotten any chances for a long time since joining ac Milan. It wasn't until the 2003-04 season that Thomasson's playing time improved, and he also scored 15 goals in all competitions, most of which were scored on the bench, so the Danes became the most efficient scorer in Serie A that year. Thomasson's excellent performance made milan coach Carlo Ancelotti call him" "the lucky star of the team, an excellent helper".
In ac Milan's final season, Thomason's form slipped, scoring only nine goals for the team. After being miraculously reversed by Liverpool in the Champions League final, Thomasson left the Rossoneri to join Bundesliga Stuttgart. From then on, Thomasson's career began to drift and begin to decline, after a season of semi-depression in Stuttgart. Thomasson came to Villarreal in La Liga in 2006, making the Dane one of the few players in international football who has scored in all four of Europe's major leagues. Except for Serie A, Bundesliga and La Liga. In fact, as early as the age of 21, Thomasson had played a season in Newcastle in the Premier League, but due to injuries and other factors, his performance was not recognized by the then head coach Daglish and the fans, and the inexperienced Thomassen only scored 4 goals for the team.
At the national team level, Thomassen is an extremely efficient goalscorer, scoring a total of 52 goals for the Danish national team, and together with Nelson is the most goalscored player in Danish history. In addition, he was twice elected Danish Footballer of the Year in 2002 and 2004. And the Danish front line he and Sander formed became the best memory of watching football in almost 20 years. After experiencing leagues in multiple countries, Thomasson returned to Feyenoord in 2008 and continued to play there for three seasons. The 2010 World Cup in South Africa became Thomasson's last memory as a professional footballer, having been injured in the group match against Japan despite scoring goals. Due to this injury, Thomasson did not even get a chance to play in the 2010-11 season. In desperation, Thomason, then 35, had to announce that he would hang up his boots.
On the day of the press conference, the Dane said emotionally: "This is a very sad day." I've been recovering for over a year, but my doctor told me it would take at least three more months and there's no guarantee that I'll recover completely. Being a professional player is one of the things that is unbeatable and I am proud of my career. Thomasson once revealed that he is a football fanatic who loves the green field. So after hanging up his boots, Thomasson did not stay away from football and he began his coaching career in the Netherlands. He chose another rotterdam team, Rotterdam's elite assistant, for the first match of his coaching career. There, he started as an assistant coach and after receiving recognition from the top, he moved to head coach in 2013.