Ireland international Sean Cronin has shaken off a knee strain to claim his spot at hooker, and regular out-half Ian Keatley shifts to outside centre to replace the injured Eoin Griffin. Keatley had been a doubt due to a calf injury.
New signing Rodney Ah You will hope to make his debut for Connacht as a replacement. The 22-year-old New Zealander recently linked up the with the province, joining from Canterbury.
Meanwhile, flanker Johnny O'Connor will make his 100th appearance for Connacht this weekend. The 30-year-old flanker from Galway has been capped 12 times by Ireland and made 50 appearances for English club London Wasps before returning home in 2007.
Sunday's match will also see Connacht full-back Gavin Duffy line out against some of his former Harlequins team-mates.
Ireland international Duffy made 110 appearances for 'Quins between 2003 and 2006, winning the European Challenge Cup in 2004 thanks to a nail-biting final victory over Montferrand.
Asked whether this is a 'must win' game for his side, Connacht head coach Eric Elwood said: "We have not put that amount of pressure on the lads. I don't think if we lose we are out, but instead we are seeing it as an opportunity.
"We have a break from Magners League duty, we are going to London, playing Harlequins, it's live on Sky Sports and it's a great opportunity in a European competition to put down a marker - that is our intention.
"We are under no illusions regarding the huge challenge we face at the Stoop, both collectively as a young team and for me as a young coach.
"But it is one we are really looking forward to. We have met Harlequins a fair few times in this tournament so we know each other pretty well. It will certainly be a test for us."
CONNACHT (v Harlequins): Gavin Duffy; Brian Tuohy, Ian Keatley, Keith Matthews, Fionn Carr; Miah Nikora, Frank Murphy; Brett Wilkinson, Sean Cronin, Jamie Hagan, Michael Swift, Mike McCarthy, Andrew Browne, Johnny O'Connor, John Muldoon (capt).
Replacements: Dermot Murphy, Ronan Loughney, Rodney Ah You, Bernie Upton, Mike McComish, Cillian Willis, Aidan Wynne, Troy Nathan.