But almost 20 minutes later - and sporting anything between 12 and 15 stitches above his left eye - Farrell returned to play an important role as England smashed the woeful Welsh in record-breaking fashion in a 62-5 mauling.
Farrell, most judges believe, was playing to confirm a place in England's 30-man World Cup squad, which will be announced by head coach Brian Ashton on Tuesday week.
Ultimately, he probably did enough to book his seat aboard England's Paris-bound flight on September 3, but not before that early scare.
Farrell said: "The doctors did a fantastic job.
"The only thought going through my mind was that I would go back on and it would start bleeding again, but that didn't happen and I was very appreciative of what the doctors did."
Farrell enjoyed an armchair ride in his role as England's defensive captain, given that Wales hardly constructed a meaningful attack due to them being horribly outmuscled by a heavyweight home pack.
He added: "When you are playing behind a pack like that and they got the dominance they did, it makes it easy for the backs.
"I just tried to concentrate on what we had been doing all week, trying to help the team move forward and get off to a good start."
Farrell might not feature against Twickenham visitors France next Saturday, although Ashton could be tempted to have another look at his midfield partnership with Leicester's Dan Hipkiss.
The World Cup hosts will unquestionably provide tougher opposition than Wales managed - England also meet them in Marseille on August 18 - and next weekend's game gives him the chance to run further checks on squad places, possibly half a dozen or more, about which he remains undecided.