Dawson suffered a knee injury in last weekend's Zurich Premiership victory over Leicester Tigers and could be sidelined for six weeks.
It means the 47-times capped scrum half will miss the back-to-back encounters with Cardiff - two matches Smith realises his side must win if they are to stand any chance of reaching the knock-out phase from Pool 6.
A late penalty handed French champions Biarritz victory over Northampton in the last round of matches. Biarritz face Ulster on Friday night and will be looking to make it three wins out of three in the pool.
The Saints opened with a win over Ulster at home, while Cardiff have yet to pick up a point. Nevertheless, Smith knows that Dai Young's men received a huge morale boost in beating Edinburgh in the quarter-fianls of the Celtic League last weekend and is wary of the threat the Blue and Blacks pose.
'Northampton is a rugby town and the people love the game. With our Heineken Cup triumph in 2000, the anticipation and expectation for this tournament is heightened,' said Smith.
'We recognise the challenge that's ahead and we're taking these next two games very seriously. We respect Cardiff and we know they have some good forwards and strike runners, so we're going to have to be careful.
'We've got to play our game. We always try to take the initiative and take teams on and this weekend will be no different.
'We played Cardiff last year in the Heineken Cup just after I joined the club, and we won that day. But it was close and we know how hard they work and how dangerous they can be.
'But the pressure doesn't weigh heavily on me, it's the sort of thing we thrive on. It's going to a close game, but it's going to be about who's mentally ready up for it on the day.'
Ian Vass comes into the side to replace Dawson, while Wales lock Steve Williams returns to the pack to face his old club after recovering from flu.
Northampton Saints: Nick Beal; Jon Sleightholme, Chris Hyndman, John Leslie, Ben Cohen; Paul Grayson, Ian Vass; Robbie Morris, Steve Thompson, Chris Budgen, Rob Hunter, Steve Williams, Andrew Blowers, Budge Pountney (captain), Mark Soden,
Replacements: Dan Richmond, Tom Smith, Jon Phillips, Simon Hepher, Johnny Howard, Oriol Ripol,