The 31-year old American centre-back was sent on loan to Spanish side Malaga last term but hardly featured and does not expect to return in 2013/14.
"I'm not holding my breath (about Malaga)," he told the Washington Post.
"I am looking for other options. I thought I was in good standing [last year] but it turned in the wrong way. I definitely need to put myself in a good place."
The USA international is contracted to Sporting for another 12 months but does not feel he will be given much game time so is keeping his options open, including the faint possibility of a MLS switch.
"I will never exclude MLS but I am not ready to leave Europe yet," he added.
Since leaving French club Metz for Belgium's Standard Liege in 2004, Onyewu joined Newcastle United on loan in 2007, prior to his 2009 shift to Italian powerhouse AC Milan. A loan with Holland's FC Twente soon followed and then did a permanent move to Portugal's Sporting in 2011 ahead of the subsequent loan to Malaga.