From Wikipedia: In August 2008, Crosas joined Celtic for a fee of £415,000 rising to £1.6 million depending on appearances, and signed a four-year contract. Barcelona retained a £1.65 million buy-back option which took effect at the end of season 2009/10. He was paraded at Celtic Park before the match with St Mirren. He made his debut for the first team as a substitute for Scott Brown against Falkirk at Celtic Park on 23 August 2008 and his first on 13 September against Motherwell at Fir Park. On 4 October, he was awarded man of the match for his performance against Hamilton Academical after completing his first full 90 minutes for Celtic. On 28 January 2009, Crosas scored in the penalty shootout of the Scottish League Cup semi-final against Dundee United. Celtic won the shootout 11-10. On 28 February 2009, Crosas scored his first Celtic goal in a 7-0 home win against St Mirren; a powerful curving shot from 30 yards, which was later voted Celtic’s Goal of the Season. He was also voted Celtic’s Young Player of the Season in his first season with the club.
In the 2009–10 season, Crosas was not initially a regular starter in midfield as Massimo Donati and loan signing Landry N’Guémo were first picks. After Donati’s transfer to Bari, Crosas got a chance to shine and forged a successful partnership with N’Guemo, becoming a staple of the Celtic starting eleven.
At the start of the 2010–11 season, Crosas fell out of favour under new Celtic manager Neil Lennon, but on 22 September he came off the bench to replace Joe Ledley in a 6-0 League Cup win over Inverness Caledonian Thistle.