Zajac began his junior ice hockey career with the Salmon Arm Silverbacks of the British Columbia Hockey League (BCHL). One of his teammates was Brady Murray, who would briefly play in the NHL for the Los Angeles Kings, and his linemate was future Calgary Flames winger Kris Chucko. In his second season with Salmon Arm, Zajac led the team in goals, assists and points, and finished second overall in the League in the latter two statistics. At season's end, he was named the MVP of the BCHL's Interior Division, and he played in the BCHL All-Star Game. His play attracted attention, and he was projected to be a potential first-round draft pick in the 2004 NHL Entry Draft. The projection proved accurate, as the New Jersey Devils would take him with the 20th overall pick. The Devils traded up to get Zajac, which "mean[t] a lot" to him, but was not a surprise, as the Devils scouted him more than any other team. Zajac opted to play college ice hockey for the University of North Dakota rather than major junior hockey in the Canadian Hockey League (CHL), as he wanted an education in addition to playing hockey. Then-Devils General Manager Lou Lamoriello commented that there would be room for Zajac on the North Dakota roster because the Devils were recalling forward Zach Parise, drafted the year earlier, to play professional hockey.