Reports in the French media have suggested the 32-year-old fly-half, who has spent his entire senior career with London club Saracens, will leave for Paris-based Racing at the end of the season.
Farrell has taken a mental health break from international rugby and will miss the upcoming Six Nations Championship after captaining England to a third-place finish at last year's World Cup in France, where he was repeatedly booed by supporters.
Should England's all-time leading points-scorer head to the French capital, it would make Farrell ineligible for England selection.
But England coach Borthwick, speaking after naming Jamie George, a Saracens teammate of Farrell, as his new Six Nations skipper told a Twickenham press conference on Wednesday: "I spoke with Owen yesterday and I think what's important is that Owen makes a decision that's right for Owen and his family.
"Will a player of Owen's calibre be missed if he chooses to play outside of England? Yes, of course.
"Do I want him in some point in the future to come back (to play for England)? Yes.
"But I also want him to do what's right for him and his family and have the experiences and the memories that that he wants to make."
Borthwick's 36-man training squad for the Six Nations features seven uncapped players, including Exeter wing Immanuel Feyi-Waboso, available to England through family heritage.
The 21-year-old is also qualified for Wales, having been born and brought up in Cardiff.
His international allegiance will remain uncertain unless and until he is capped by either country, with England opening their Six Nations campaign against Italy in Rome on February 3.
"Manny's really impressed us, not just on the pitch," said Borthwick. "We can all see the power he brings, the speed he brings. He finds a way through contact when there doesn't appear to be a way through there."
Feyi-Waboso played for Wasps, one of three English Premiership clubs that went out of business last season, before joining Exeter.
"He's a real mature guy," added former England captain Borthwick. "With his medical studies and then to go through what he went through at Wasps, moving down to Exeter, restarting from a rugby perspective and restarting from a studies' perspective, a real sense of maturity with a young man. He's really excited by this step.
"When I rang him yesterday (Tuesday) and spoke to him, he said he had just finished his university studies for the day.
"I said 'I'm announcing my Six Nations squad tomorrow and naming you in it'. I could sense that excitement and buzz down the phone, he was thrilled.
"And that gave me a real energy and a real buzz as well."
