The defending champions were in superb form and outscored the Premiership outfit by eight tries none, after holding a 14-3 lead at half-time.
But despite turning on the style in the second half, which they won 38-0, Cullen is not reading too much into his side’s performance.
“We just need to be careful that we’re not getting too far ahead of ourselves,” he said after the match.
“We’ve played one game and no one else has played yet in Europe. There’s a lot of very good teams in Europe at the moment.
“Sometimes in Europe, you only need to be a little bit off and you come unstuck. Sometimes then the score can get out of control.
“We know that ourselves because we got beaten by Wasps 51-10 in the Ricoh a number of years ago and we got pretty heavily beaten here as well. It can be a little bit misleading at times.”
Cullen praised his charges for their clinical performance but feels there were some crucial moments in the match which helped their cause.
“We talked a lot in the past about every point counting,” he said.
“It was good to keep playing until the very end. Big moments obviously just on half-time, when they lose a player (Lima Sopoaga) to the bin and we go on to score just before half-time (Luke McGrath’s first try).
“Then score two minutes into the second half (through James Lowe).
“We managed to hold onto the ball a bit better than we probably did last week against Munster.
“We coughed up a lot of ball (against Munster) and had very little possession. It was nice to be on the other side of the possession stats.”