The Bhoys coach was reportedly in the stands to watch the game between PSV Eindhoven and Ajax on Sunday night and paid close attention to the display by the 23-year-old forward, who is under contract in Amsterdam until 2015.
Celtic are due to play Ajax in the Champions League and so it was a fruitful exercise for the Northern Irishman, with Ajax losing 4-0.
Sigthorsson has suffered with injury since moving to the Dutch giants in 2011 and has made only 34 league appearances, but he has scored 17 goals already and caught the eye of a few top clubs around Europe.
His record with Iceland is equally as impressive, with 11 goals in 17 games since making his debut in 2010.
Whether or not Ajax would allow the youngster to leave is another question, after Christian Eriksen was sold to Tottenham in the summer.