Former London Irish wing Arundell has rebuilt his club career with the Parisian giants in the wake of the Exiles folding last season.
The 21-year-old went straight to his new Paris home after featuring for England at the World Cup in France.
Arundell has special dispensation to keep playing for England this season despite moving abroad, given Irish's untimely and damaging demise.
England cannot select overseas-based players save for in exceptional circumstances, and Arundell will likely be off-limits if he stays in France beyond next summer.
The RFU want to bring him back to an English club as soon as next season, with Bath and Gloucester in the frame.
Former England boss Lancaster traded Leinster for Racing in the summer, and has now revealed his drive to help Arundell flourish for country as well as club.
"From my point of view, any international player who plays in the Six Nations, I’m never going to compromise their chances to be successful for their national team," said Lancaster.
"We’ll see what happens with Henry. It’s early days for him and there’s a decision to be made.
"Whichever way he goes, I’ll support him, and try and help him develop, for sure.
"He's probably been here six weeks, and he’s been great in training, he has a very professional attitude and he's very mature for a 21-year-old; mature in terms of his outlook in life and what he wants to achieve.
"We’ve had a chance to play him on the wing and at 15. Both options are open.
"His athleticism obviously stands out but I’ve been impressed with his ball distribution, his kicking game, his aerial game and defensively he’s switched on. He’s definitely had a good start and I think he’s enjoying it."
Lancaster was crucial to formulating England’s overseas player rules when taking the Twickenham helm in 2011, to help keep the best talent on home soil.
Still backing that rule, Lancaster nevertheless is ready to work with current boss Steve Borthwick to help Arundell's Test match cause.
"It’s a very tricky one isn’t it? I go back to 2011, I got the interim England job, and there were decisions to be made about players who were in France," he said.
"And the RFU has held the line with, obviously, Premiership Rugby squarely behind, because they want to keep their homegrown talent in England.
"I 100 per cent can agree with why that’s important to England and to the Premiership.
"It obviously is getting harder to do that on a more consistent basis. Because players are wanting to grow as well, and wanting to take opportunities."
Racing told Lancaster that they had no Plan B when approaching him to take charge, underscoring the esteem he now commands at rugby's very summit.
Lancaster helped oversee a period of major dominance at Leinster, as the Irish province claimed four URC titles and one Champions Cup triumph.
Now installed in Paris, Lancaster has not shied away from a tilt at a Top 14 and Champions Cup double, to add to the 2018 sweep in Dublin.
"We’ll see what happens with Henry... it’s early days for him and there’s a decision to be made"
Stuart Lancaster on Henry Arundell's future
Harlequins will face Lancaster’s Racing in Paris on Sunday, and will be left in no doubt as to the scale of the challenge in light of their lofty ambitions.
"I’ve actually asked the question to all the players and all the staff, 'what’s more important?' Because the Top 14 is huge here in France, but clearly I’d be doing both competitions a disservice if I said one was more important than the other," said Lancaster.
"I’ve had the privilege of coaching a team which did the double in 2018, the sense of fulfilment the players and coaching staff got from that particular year is probably unparalleled in any experience in my career, so the goal is to win both. To try and win both.
"The mindset is to go out and win the next game that’s in front of you then hopefully that gets you somewhere close towards the end of the season.
"Last year we lost in a semi-final and then a final, but in 2018 we managed to do the double. I hope it’s more like 2018 than last year."