The Saints were second best throughout the contest at Twickenham, as they trailed 41-3 at half-time and went on to concede a total of nine tries.
Captain Hartley admits his side deservedly lost the match, but he insists they will stick together as they bid to bounce back from the disappointment.
"There were a lot of people writing us off before this tournament and a lot of people's prophecies were confirmed today," Hartley told BBC Northampton. "But we will stay together, keep our heads down and keep working."
The England international, who signed a new contract with the Saints during the off-season, insists the manner of the defeat was certainly a wake-up call, but he is convinced the loss will not define their season.
"It's a very grounding and humbling experience. Pre-season was going really well and we've had a bit of a shock today," he added. "I've been in this game long enough to know that one game doesn't define you, and I'm not going to let this derail our season."
Northampton finished seventh in the top-flight last season, 14 points adrift of the play-off positions, and they are next in action when they play host to Leicester Tigers at Franklin's Gardens next weekend.