The former Rugby League star was rested from Sarries' 19-8 win over London Irish and will resume training with the national squad in Bath on Monday.
Farrell has made only seven first team starts since his injury-plagued move into union from Wigan two seasons ago.
But Gaffney insisted he "definitely justifies selection" and expects Brian Ashton to hand him the number 12 shirt when the England side is named, with Gloucester's rejuvenated Mike Tindall his likely midfield partner.
And the Australian - who controversially started the season by playing Farrell in the back row - admitted he was startled at the rate of the former Great Britain skipper's progress as an inside centre.
"I'm delighted for Andy Farrell that it looks like he'll start against Scotland," said Gaffney.
"I must admit I didn't think he would progress as quickly as he has done. It's been a delight seeing how far he's come.
"He's got the ability to play the game going forward and laterally, and there aren't many players like that. He's made tremendous strides and definitely justifies selection in the national side.
"When you've got a left-footed kicker at 12 it's a huge plus if you're going with a right-footed fly-half so he's got a lot going for him in that respect.
"I thought for a while that he might be exposed defensively at 12. Although he's a good tackler it's a difficult position to defend in but he's developed very well in that area too."
With Farrell starting, a place on the bench looks most likely for Exiles' veteran Mike Catt, who came through his first game since December 15 at Vicarage Road.
The World Cup winner played a full 80 minutes but could not do anything to halt a powerful performance from the Saracens pack who controlled the game.
"Catty put his hand up," said Irish boss Brian Smith.
"On a losing side he played well and if Brian Ashton wants someone in midfield then Mike put his best foot forward today."