Negotiations could be wrapped up by noon on Wednesday, but if Lech Poznan do not accept Claasen's demands the move to Poland may fall through.
Claasen would join new Poznan signings Simon Pawlowski, Barry Douglas, Dimitrije Injac, Matthew Gostomski and Paulus Arajuuri if the deal is completed.
Claasen impressed during trials in England for Hull City and Blackpool, but his lack of international caps meant securing a work permit would have been difficult for the talented midfielder who can play in both central midfield and out on the wing.
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Meanwhile, other South African overseas based players are expected to turn out in the Uefa Europa League playoffs on Thursday night.
Anele Ngcongca should play for Genk as they face FH Hafnarfjardar, returning following the injury debacle which kept him out of Bafana Bafana's international against Nigeria as well as Genk's league encounter with Lokeren last weekend.
Dino Ndlovu could make his first appearance for Maccabi Haifa after recently declaring himself fit. The Israeli outfit will host Romanian club FC Astra Ploiesti in their playoff encounter, with hopes of attaining a spot in the continental competition this season.