The Scottish champions play their first competitive game of the season in Iceland tomorrow, in the first leg of their Champions League second qualifying round tie.
Their opponents KR were stunned by struggling Thor in their last game in the league, and Sigurjonsson believes his side have been guilty of thinking too far ahead.
"Perhaps the thought of playing Celtic has been disrupting our form," he said. "I don't think our form has been too bad - we just let ourselves down in our last game. Although we know it will be tough, we are confident we can give an excellent account of ourselves."