HITC Sport understands a host of English clubs are chasing the Dutch striker.
Castaignos has been highly-regarded for some time having come through the ranks at Feyenoord and making his first-team debut before he was 17.
The skilful forward scored 15 goals in his first full season in the Feyenoord first team, bringing him to the attention of a host of Europe's top clubs, and he subsequently joined Inter Milan.
However, the spell at Inter did not work out as he made just appearances for the club before leaving the San Siro to return to Holland with FC Twente.
Castaignos spent three seasons at Twente, scoring 37 goals in the Eredivise before Eintracht Frankfurt snapped him last summer.
The 23-year-old netted five goals and three assists in 21 games for Eintracht last season and the German club could now face a fight to keep hold of Castaignos before the transfer window closes.
Premier League duo Southampton and Stoke City are both keen on the former Netherlands Under-21 international as they look to add to their attacking options, while Newcastle United, Norwich City and Cardiff City are all ready to move for Castaignos as they look to bring in a new striker.