Onyewu has spent six months on loan at Twente from Milan, but the Dutch club has decided to decline the option to buy him.
The American international spent six months at the De Grolsch Veste, making 14 appearances, but coach Michael Preud'homme has decided that he is now surplus to requirements.
The Eredivisie runners-up confirmed the news on their official website, and the 29-year-old will now need to either fight for a place at Massimiliano Allegri's Scudetto winners or look for a new challenge elsewhere.
The former Newcastle and Standard Liege stopper will find it difficult to break into the Milan squad, with new defensive additions Taye Taiwo and Phillippe Mexes increasing the competition for places.
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