 During O'Neill's first three years at Celtic, Valgaeren was first choice in a three-man defence consisting of himself, Johan Mjällby and Bobo Baldé.He played regularly in Europe, scoring in a 4–3 win at Celtic Park against Juventus in the 2001–02 Champions League campaign, and was part of the Celtic side that reached the 2003 UEFA Cup Final.However, Valgaren struggled with injuries in his later years at Celtic and did not play regularly.