Despite the likes of Ricky van Wolfswinkel and Nathan Redmond impressing in the first half, Chris Hughton's men fell to a second-half strike to end a disappointing visit to USA
The Canaries lost 1-0 to San Jose Earthquakes on Saturday and ended their tour of USA in a similarly sour fashion in Portland.
In the 53rd minute, a lax pass from Norwich central defender Ryan Bennett allowed Kalif Alhassan to gain possession and the Ghanaian winger sprinted past Russell Martin before squaring the ball to Valencia, who slotted his shot under visiting goalkeeper Carlo Nash.
Norwich had the better of the first half, with Ricky van Wolfswinkel, Robert Snodgrass and Nathan Redmond leading the way for the Premier League club, but, after going behind to the Timbers, Chris Hughton's side rarely tested their hosts.
Substitutes Jacob Butterfield and Luciano Becchio both had chances to score in the final 20 minutes but were unable to beat Portland goalkeeper Jake Gleeson.
The result meant Norwich ended their USA tour with one win and two defeats, having secured their only victory in a quickfire 3-0 beating of second-tier Mexican outfit Dorados on July 18.