Whilst Rees acknowledges there were many positives from the victory, the Welsh hooker wasn't happy enough with the performance of his pack and he is looking to put that right this weekend.
"Northampton are one of the best teams in the Premiership and reached the Heineken Cup final last season. They've got a big pack and really rely on their set-piece to dominate games - it's a massive challenge for us up front," he said.
"Castres had one of the biggest packs I've ever come up against and it was a bit of an eye opener in terms of the scrum. We came unstuck there and have to be spot on this week because Northampton will look to squeeze us and get penalties there.
"Dylan Hartley is massive for them - if he plays well the whole team plays well. He will be disappointed not to be England's first choice during the World Cup and will want to make a statement but I'm looking forward to the challenge.
"You play against these players a lot for your country and it's good to face them in the Heineken Cup. But it's not just Dylan Hartley that we need to keep quiet.
"Their whole pack from one to eight is really important - Soane Tonga'uhia, Brian Mujati, Courtney Lawes, Tom Wood and Roger Wilson are massive for them.
"We have experience in the Heineken Cup, we've won our first home game and it's about performing at Franklin's Gardens on Friday night.
"It's been a long time since we achieved anything in Europe. We've got to get out of the group stages and make something of this Heineken Cup."