The County manager had pursued the midfielder - who was keen on a move to England - for some time, but the long-awaited switch has been a resounding failure due to a groin injury for the Mali-born left-sider.
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"He has been recommended to rest for 45 days so that is it," Thordarson told the Nottingham Evening Post.
"Unfortunately that has turned into a difficult signing for us. I have not seen a great deal from him. We saw a few minutes out here (against Peterborough). He is good on the ball but that groin problem he picked up he has not recovered from, so that is it."
Sissoko has also struggled to settle in England and has made frequent visits back over the Channel in order to try and resolve his homesickness.
County take-on Bury on Saturday in one of the most important games in the club's history, but it is not yet clear whether the Magpies' other recent absentee - Eugene Dadi - will be available to play.
Striker Dadi was another signing in January but has struggled with injuries and has fell out-of-favour recently, not featuring in the first-team since a 2-0 defeat at Northampton back in March.
For all the relegation permutations ahead of Saturday's crunch encounter, stay tuned to NCM.