Clasie made his professional debut for Excelsior on 15 August 2010, when he was part of the starting line-up in the Rotterdam derby victory against Feyenoord (3-2). On 31 July 2011 he made his debut for Feyenoord in a friendly game against Málaga. Because of his good play and his tenacity he soon became one of the most popular players among the supporters. His style of play has earned him the label of the 'Dutch Xavi' and has also been touted as a replacement for Andrea Pirlo.
On November 2011, two months after his debut for Excelsior, he was selected for the Netherlands U21 squad. On February 2012, Bert van Marwijk, the coach of the first team of the Netherlands, pronounced his interest in Clasie. He let him know that he is on the stand-by list for Euro 2012.