Glenn joined Sunderland on a free transfer after leaving Sheffield Wednesday following the end of the 2017-18 season.
The Dutchman signed a two-year deal at the Stadium of Light in July 2018.
The 34-year-old started his career with Feyenoord, progressing through the ranks alongside the likes of Robin van Persie before breaking into the first team in 2001.
After loan spells with Excelsior and De Graafschap, his first taste of English football came in 2005 when he joined Cardiff City on loan ahead the 2005-06 season.
One year later he signed a permanent deal with the Bluebirds and he went on to make over 100 appearances for the Welsh side, while also playing a key role in their memorable run to the 2008 FA Cup final.
Loovens spent four seasons at Celtic between 2008 and 2012, winning the Scottish Premier League, the Scottish Cup and the Scottish League Cup during his time north of the border.
After a spell with Real Zaragoza he returned to England in 2013, signing for Sheffield Wednesday where he would go on to make 150 appearances and become club captain.
The Dutchman has also represented his country at senior level, earning two caps.