Loovens was the subject of transfer interest for most of summer 2008, with Old Firm rivals Celtic and Rangers emerging as leading candidates to secure his signing. Walter Smith, manager of Rangers, declared his interest in Loovens and made an offer for his services but their valuation of the defender was rejected by Cardiff. On 16 August 2008, Loovens joined Scottish champions Celtic on a four-year contract for a fee of around £2.1 million Loovens stated that Parkhead was his preferred destination as he will get the chance to play in the Champions League. He made his Celtic debut on 23 August 2008, in a 3–0 win over Falkirk at Celtic Park. His first goal came on 23 September 2008 in a Co-op Insurance League Cup tie, also at Celtic Park, as Celtic beat Livingston 4–0.
Loovens scored the winning goal against Hearts in injury time on 20 September 2009, to send Celtic top of the table. He scored an own goal in a 1–3 defeat against Rangers in October 2010 and was criticised for his performance in Celtic’s next game, a League Cup win over St Johnstone. Loovens paid for his poor form, as he was dropped for the following match, also against St Johnstone.