The 28-year-old, a regular starter at La Liga team Real Zaragoza last season, told CSKA he wanted to continue his career in Brazil.
"We see no reason to keep him in Bulgaria against his wish," CSKA's chief executive Ventsislav Zhivkov told reporters on Thursday.
After signing last month, Diogo, capped 23 times by Uruguay, had posed for photographers holding the red CSKA shirt, saying he was "proud to sign for Bulgaria's Real Madrid."
Diogo moved from Argentine side River Plate to Real Madrid in 2005 but was given few opportunities in the first team at the Bernabeu.
Local media reported that Diogo will most probably move to Palmeiras, coached by World Cup winner Luiz Felipe Scolari.
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