It was 12 days ago that Owen Farrell blindsided Wasps' full-back Charlie Atkinson, but the ripples from his chin-high hit are still being felt.

Now, ruled out of Saturday' Champions Cup quarter-final, Farrell finds himself playing the unusual role of training-ground impressionist rather than on-field virtuoso.

Farrell has spent the week as Saracens' St Albans base, replicating the patterns run by Leinster fly-half Johnny Sexton. There are few better qualified.

The pair first lined up opposite each other in England's 30-9 win over Ireland at Twickenham in 2012. At one point in that match, Farrell, then just 20, had Sexton pinned to the floor.

"I told him that just because his old man was a hard man, that didn't make him one too," remembered Sexton in his 2013 book, referencing Farrell's dad, rugby league legend Andy.

After the match, Farrell visited the Ireland dressing room to shake hands and swap shirts regardless. It has been the same ever since, through club and country battles and two British and Irish Lions tours as team-mates. A rivalry underpinned with respect.

Whatever pre-game insights he brings, Farrell, who also won 37 of his 83 England caps under now-Leinster coach Stuart Lancaster, won't add to their shared history on Saturday.

Farrell called the plays and landed the kicks as his team inflicted Leinster's last defeat, now more than a year ago in the final of the same competition.

But former England half-back partner Danny Care believes it will be without the ball that the defending champions feel Farrell's absence most keenly.

"They will miss his noise, his line speed, his energy - he doesn't stop screaming at people to get off the line and hit people," Care told Rugby Union Weekly.

Alex Goode, the most likely candidate to replace Farrell, operates at a different volume.

A sublime runner, he hurts teams with attacking stealth rather than high-decibel defence.

"My game has always been based around thinking and trying to outsmart the opposition," said Goode earlier this week.

"I think there are elements of Owen's game - like his control - which I would like to implement. I think the way that we attack the line and the way we move and try and attack is slightly different."

The change didn't do Saracens much harm at the same stage of last season's campaign.

Farrell was attending the birth of his first child on the morning of Saracens' last-eight meeting with Glasgow.

Despite Farrell phoning from the maternity ward 45 minutes before kick-off claiming he might still be able to play, Saracens director of rugby Mark McCall stuck with his decision to move Goode from his usual full-back role to 10.

Goode pulled the strings with precision in a 56-27 win.

Saracens' hopes may not rest on any one individual though, however influential.

This meeting with Leinster has been their focus since a critical 24-hour period at the start of the year.

On the evening of Saturday 18 January, Premiership Rugby confirmed that the club, winners of four of the previous five domestic titles, would be relegated as punishment for breaching the top flight's salary cap.

The following afternoon, Saracens found themselves four points and a man down after 40 bruising minutes and a Will Skelton red card against French powerhouses Racing 92.

Defeat would have ended their European campaign and any competitive interest in their season. Given the uncertainty over the future of their all-star squad at the time, it could have ended an era.

According to coach Alex Sanderson, Farrell told his team during the interval they could "either lie down and roll over or stand up and rise."

Saracens, depleted in number but with a surfeit of spirit, won 27-24 to book into the quarter-finals.

Harlequins wing Chris Ashton, who spent five years as part of the Saracens dressing room between 2012 and 2017, is not putting anything past his former team-mates.

"I think it will be a lot closer than people think," he added on Rugby Union Weekly.

"If they are within five going into the last 20, I think it is game on. I think they have more than enough to win."

And if another half-time pep rally is needed, they also have Farrell ready, waiting and willing to play his part.

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