Fly-half Ben Botica, who marked his second senior appearance for the club last week by kicking 18 points, will make his starting debut, replacing Nick Evans who was also taken off last week with an ankle injury.
The other change at the Sportsground in Galway sees Sam Smith coming in for Ugo Monye on the wing after the England international also sustained an ankle injury last weekend.
Connacht's coach Eric Elwood has made one change to his starting XV following their 19-10 victory over Zebre last weekend.
Hooker Ethienne Reynecke, who injured his calf in the closing stages of that game, will be replaced by Adrian Flavin in the front row.
In the backs, Dave McSharry earns his 20th cap for the province, and retains his midfield partnership with Eoin Griffin.
On the replacements bench, Brett Wilkinson returns to the squad from B&I Cup duty last week. He joins Jason Harris-Wright and Ronan Loughney as front row cover.
Dave Moore and Mata Fifita, both of whom made an appearance in last week's opening Heineken Cup fixture, are joined by Miah Nikora in the replacement backs.
Connacht: 15 Robbie Henshaw, 14 Tiernan O'Halloran, 13 Eoin Griffin, 12 Dave McSharry, 11 Fetu'u Vainikolo, 10 Dan Parks, 9 Kieran Marmion, 8 George Naoupu, 7 Willie Faloon, 6 John Muldoon, 5 Mike McCarthy, 4 Michael Swift, 3 Nathan White, 2 Adrian Flavin, 1 Dennis Buckley.
Replacements: 16 Jason Harris Wright, 17 Brett Wilkinson, 18 Ronan Loughney, 19 Dave Gannon, 20 Johnny O'Connor, 21 Dave Moore, 22 Miah Nikora, 23 Mata Fifita.
Harlequins: 15 Mike Brown, 14 Tom Williams, 13 Matt Hopper, 12 Jordan Turner-Hall, 11 Sam Smith, 10 Ben Botica, 9 Danny Care, 8 Nick Easter, 7 Chris Robshaw, 6 Maurie Fa'asavalu, 5 George Robson, 4 Ollie Kohn, 3 James Johnston, 2 Rob Buchanan, 1 Joe Marler.
Replacements: 16 Dave Ward, 17 Mark Lambert, 18 Will Collier, 19 Charlie Matthews, 20 Tom Guest, 21 Karl Dickson, 22 Rory Clegg, 23 Seb Stegmann.
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