Claasen injured his knee in pre-season training last July and has spent six months in rehabilitation. He returned to training towards the end of last year and over the past few weeks has had no reaction to the injury.
The 22-year-old would have been a strong contender for Bafana's Nations Cup squad, with coach Gordon Igesund talking up his qualities after he took over the job. He had also been invited for trials with English Premiership club QPR.
But despite trying numerous different types of treatment to get the knee ready in time, including shockwave therapy, he has fallen short by a few weeks.
It is certain now too that he will spend the remainder of the season with Belgian strugglers Lierse, but if he can prove his fitness in the second half the campaign then a move to a bigger league in Europe is certainly on the cards.
Lierse are up against Waasland-Beveren on Saturday and Claasen could be included by coach Hany Ramzy.
Also back in action will be Darren Keet for Kortrijk at home to Germinal Beerschot, but Ayanda Patosi will be missing Lokeren through injury as they take on OH leuven and Genk's Anele Ngcongca is at the Nations Cup.