England's David Strettle was injured scoring a fourth-minute try as Harlequins and Scarlets opened their LV= Cup campaign with a 15-15 draw.
Strettle, released to play for his club when Martin Johnson reduced his original 32-man squad for the Twickenham clash with Australia to the match-day 22, continued briefly after treatment.
But clearly struggling, he was replaced by George Lowe nine minutes later in his side's opening pool match at The Stoop.
Harlequins added a further try from lock George Robson and a penalty and conversion from fly-half Rory Clegg.
The Welsh side replied with tries from centre Gareth Maule, left-wing Andy Fenby and a penalty and conversion from number 10 Rhys Priestland.
Scarlets rested player-coach Simon Easterby and key Australian number eight David Lyons despite also being without their Welsh international contingent of Stephen Jones, Matthew Rees, Dafydd Jones, Martin Roberts, Jonathan Davies and Mark Jones, out for three months with a knee injury.
But they provided stiff resistance after being on the receiving end early on.
Harlequins should have gone ahead inside the first minute but left-winger Josh Drauninui dropped the ball after Clegg found him over the try-line with a pinpoint crossfield kick.
A minute later Strettle was forced into touch a metre out following a fine handling move involving Clegg, centre Nils Mordt and full-back Mike Brown.
But Drauinui made amends in the fourth minute, providing the final pass for Strettle, who switched wings to finish off the attack and Clegg converted from the touchline.
Priestland, the 22-year-old fly-half replacement for Stephen Jones, orchestrated the Scarlets fightback, despite landing only one of his four first-half penalty attempts.
His pace troubled Quins, without England players Ugo Mone, Danny Care and the injured Nick Easter, and he provided the final pass to send in Maule for a 25th-minute try, before converting to put the Welsh side 10-7 ahead.
Harlequins, who also lost Mordt to injury in the 32nd minute, suffered another blow when number eight Tom Guest, Easter's deputy, was yellow-carded two minutes before half-time for killing the ball at the breakdown.
Priestland opened up the Harlequins defence at the start of the second half with a shot kick-ahead and catch but failed to capitalise with men outside him.
But the Welsh side struck again in the 45th minute when Fenby snatched up a loose ball, lost in contact by Quins, 45 metres out and raced clear, aoiding the despairing dive of scrum-half Steve So'oialo on his way to the line.
Priestland was again off target with the conversion and the home side cut the deficit to five points when Clegg landed his second penalty five minutes later.
Harlequins hit back to level the scores when Robson forced his way over in the 55th minute following a spell of pressure and both sides squandered chances to win it in the closing stages.
Clegg missed a 75th-minute penalty from 40 metres before Scarlets full-back Morgan Stoddart knocked on with the last touch of the match as he chased a kick over the Quins line.
Tries: Strettle, Robson
For the Scarlets:
Tries: Fenby, Maule
Harlequins: 15 Mike Brown, 14 David Strettle, 13 Gonzalo Tiesi, 12 Nils Mordt, 11 Josh Drauniniu, 10 Rory Clegg, 9 Steve So'oialo, 8 Tom Guest, 7 Neil McMillan, 6 Chris Robshaw (capt), 5 George Robson, 4 James Percival, 3 James Johnston, 2 Chris Brooker, 1 Aston Croall.
Replacements: 16 Matt Cairns, 17 Mark Lambert, 18 John Andress, 19 Lewis Stevenson, 20 Chris York, 21 Karl Dickson, 22 Waisea Luvenyali, 23 George Lowe.
Scarlets: 15 Morgan Stoddart, 14 Joe Ajuwa, 13 Gareth Maule, 12 Rob Higgitt (c), 11 Andy Fenby, 10 Rhys Priestland, 9 Tavis Knoyle, 8 Josh Turnbull, 7 Richie Pugh, 6 Rob McCusker, 5 Damian Welch, 4 Vernon Cooper, 3 Deacon Manu, 2 Ken Owens, 1 Phil John.
Replacements: 16 Jamie Corsi, 17 Emyr Phillips, 18 Simon Gardner, 19 Aaron Shingler, 20 Johnathan Edwards, 21 Gareth Davies, 22 Daniel Newton, 23 Scott Williams.
Date: Sunday, November 8
Venue: The Stoop
Kick-off: 15.00 GMT
Referee: Tim Hayes (Wales)
Assistant referees: Ken Stewart (England), Andrew Watson (England)
Television match official: Roy Marfleet (England)