Ulster have made two changes for Saturday's Investec Champions Cup Pool 2 match at Harlequins, who welcome back fit-again Joe Marler.

England prop Marler returns from injury in one of three changes for Quins.

Second row Irne Herbst and flanker Chandler Cunningham-South are also recalled for the hosts, who have qualified for the knockout rounds.

Marler injured his arm in Quins' Premiership win over Gloucester earlier this month but was named in Steve Borthwick's England squad for the Six Nations.

A win for fourth-placed Ulster will secure their spot in the last 16. If Ulster lose to Quins and Racing beat bottom-placed Cardiff, Ulster would drop into the Challenge Cup round of 16.

Were Cardiff to beat Racing with a bonus point, they could qualify if Ulster get nothing from their trip to Quins.

If teams are equal on points, pool position will be decided by points difference.

Ulster, who were comprehensively beaten at home by Toulouse last week, have included all six of the Irish internationals - Tom Stewart, Tom O'Toole, Iain Henderson, Nick Timoney, Jacob Stockdale and Stuart McCloskey - who were named in the Six Nations squad on Wednesday.

Rea comes in for Ewers, who suffered a concussion against Toulouse, while McCann has recovered from an ankle problem to take Reffell's place at openside flanker.

Line-ups

Harlequins: Green; David, Beard, Esterhuizen, Anderson; Smith, Care; Marler, Walker, Collier, Herbst, Hammond, Cunningham-South, Evans, Dombrandt (capt).

Replacements: Riley, Baxter, Lewis, Launchbury, White, Porter, Anyanwu, Joseph.

Ulster: Lowry; Baloucoune, Hume, McCloskey, Stockdale; Burns, Cooney; Kitshoff, Stewart, O'Toole, Treadwell, Henderson (capt), Matty Rea, McCann, Timoney.

Replacements: Andrew, Warwick, Moore, Sheridan, Rea, Nathan Doak, Marshall, Addison.

Referee: Mathieu Raynal (Fra)