Several Premiership and Scottish Premier League clubs are aware of Riveros' availability from Paraguay's Libertad.
The 24 year-old carries a Spanish passport and played against England in last year's World Cup. Bolton Wanderers and Manchester City are among clubs being mentioned as possible destinations for the free-scoring midfielder.
Bolton Wanderers and Newcastle are being linked with Atalanta striker Christian Vieri.
After two knee operations last year, Vieri crowned his successful comeback on the weekend with a goal against Siena.
AS says Bolton, Newcastle, Torino and Villarreal have all made contact with the 33 year-old's representatives about next season.
Bolton are also keen on AC Milan midfielder Cristian Brocchi.
Brocchi is mulling over his future at AC Milan and it's reported that vice-president Adriano Galliani won't stand in his way should he ask for a move in the summer.
Newcastle, Bolton, Villarreal and Torino are all monitoring developments.