With Toulouse holding a two-point lead in the pool, Quins will need to beat Connacht and have Gloucester do them a favour against the Top 14 outfit if they are to clinch an automatic spot in last eight.
However should Toulouse win, then Quins can still clinch one of the two best runners-up spots.
Conor O'Shea's men know that nothing less than a victory will do if they are to reach the quarter-finals but Connacht are not about to make things easy for Quins, with coach Eric Elwood opting to name a full-strength side for the match.
Ronan Loughney replaces the benched Dylan Rogers at prop in the only change following the 24-3 defeat to Toulouse last time.
Quins have made five changes for the trip to Galway - with four of the alterations coming in the pack.
Joe Marler and Chris Brooker both come into the front row for the benched Joe Gray and Mark Lambert, while Tomas Vallejos and George Robson will team up in the second row.
The only change in the backs comes on the wing with Sam Smith coming in for Tom Williams, who is missing as his wife is due to give birth.
Connacht: 15 Gavin Duffy, 14 Fetu'u Vainikolo, 13 Kyle Tonetti, 12 Dave McSharry, 11 Tiernan O'Halloran, 10 Niall O'Connor, 9 Paul O'Donohoe, 1 Brett Wilkinson, 2 Adrian Flavin, 3 Ronan Loughney, 4 Michael Swift, 5 Mike McCarthy, 6 John Muldoon, 7 Ray Ofisa, 8 George Naoupu.Replacements: 16 Ethienne Reynecke, 17 Dennis Buckley, 18 Dylan Rogers, 19 Mick Kearney, 20 Eoin McKeon, 21 Dave Moore, 22 Miah Nikora, 23 Henry Fa'afili.
Harlequins: 15 Mike Brown, 14 Ugo Monye, 13 Matt Hopper, 12 Jordan Turner-Hall, 11 Sam Smith, 10 Nick Evans, 9 Danny Care, 1 Joe Marler, 2 Chris Brooker, 3 James Johnston, 4 Tomas Vallejos, 5 George Robson, 6 Maurie Faasavalu, 7 Chris Robshaw, 8 Nick Easter.Replacements: 16 Joe Gray, 17 Mark Lambert, 18 Tim Fairbrother, 19 Ollie Kohn, 20 Luke Wallace, 21 Richard Bolt, 22 Rory Clegg, 23 Tom Casson.
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