Paul Lambert's created several chances - with winger Anthony Pilkington slotting wide when clean through just before United went up the other end and took the lead with a header from Anderson on 68 minutes. Pilkington also had a deflected shot come back off the post before Danny Welbeck wrapped up the three points with a late close-range effort.
"I think if you ask any of the (United) players in their dressing room and they'd say they have been in a right good game," Tierney said on BBC Radio Norfolk.
"It was 0-0 at half-time; we were working them very hard; hitting them on the break on numerous occasions.
"On any other day some of those go in for you and, who knows?"
Tierney added: "I think we have got to take a lot of confidence from that for the rest of the season.
"Hopefully, we can reach those heights in every game and if we do that, we should not do too badly."