But Thordarson is convinced the Argyle man will come good and prove to be an excellent recruit.
"Simon hasn't played a great deal of first-team games and showed he was a bit rusty," he told the Plymouth Herald. "But if he had been playing regularly at Plymouth I wouldn't have been able to have got him here.
"We will see him improve and get stronger, and we will see his quality come through.
"I have brought him in with two options in mind. He can play centre-back and in midfield."