Sexton has recovered from the quad injury which kept him out of a starting berth in the victory over Munster at the weekend just in time to reclaim the No10 jersey for the first fixture in a pool which Sexton is calling 'the group of death'.
"It's a very tough group," said the Irish playmaker, who guided Leinster to Heineken Cup glory in 2009 after stepping into the breach following Felipe Contepomi's injury heartbreak.
"I think the two toughest groups will be ours and Munster's so to get out of them and into the knockout stages will be an achievement in itself.
"But if we can get out of our group we have got every chance of going on to win the competition. But we're not getting ahead of ourselves. The first target is to get out of the group and that's going to be pretty tough."
Leinster clinched Europe's biggest prize for the first time two years ago, beating double Heineken Cup Champions Leicester at Murrayfield, but last season's adventure was halted by Toulouse in the semi-finals as the French side went on to win their fourth crown.
To reach the knock-out stages this season, the Irish province face the daunting prospect of taking on Top 14 Champions Clermont Auvergne, Racing Metro and English side Saracens in the pool rounds.
Sexton is in pole position to go into next summer's Rugby World Cup in New Zealand as Ireland's first choice fly-half but the 25-year-old is much more focused on Leinster's immediate challenge.
"I don't really view the World Cup as happening this season but it's obviously important to have a good season this year and put yourself in the frame for the squad. But before then we've got the Heineken Cup and the Six Nations which are more important to me this year than the World Cup," added Sexton.
First up for Leinster are Racing-Metro, who are making their Heineken Cup debut in the fixture on Saturday. The Irish side then travel to England to face Saracens at Wembley Stadium on Saturday, October 16.