And the American four-time winner of the Tour de France implied there could be an opening for Millar at his mighty US Postal team.
Millar, whose contract with France's top team Cofidis expires at the end of this season, recently complained that his team's tactic of not designating a team leader for major races had instilled a "mercenary-like" environment within the team.
Cofidis riders basically race for their own account, winning personal bonuses and UCI points which allows the team to maintain its position in the world team rankings which in turn gives easy access to all the major Tours.
As a result, Cofidis is the only French team in the world's top ten and enjoys automatic entry to all the major races.
US Postal, who this year could see Armstrong pick up a fifth straight Tour de France, employ an opposite set-up in which all riders sacrifice themselves for the team leader.
And the American admits he can understand why Millar is mulling over his future.
"David is a friend of mine, and he's a brilliant rider, very talented," the American said in a report in L'Equipe Monday in response to Millar's recent outburst.
"It's true that sometimes he lets himself get distracted easily and he's now at an age where he must be saying to himself: 'it's now or never'.
"But it's also true that Cofidis's set-up is not exactly suited to him and I'm not saying that because I had a very bad experience with the team."
Armstrong, who survived testicular cancer to come back and win his first Tour in 1999, was signed to Cofidis in 1996 by legendary directeur sportif Cyrille Guimard, who had won seven Tours as a team manager.
However the 32-year-old Texan was allowed to leave the team when it became clear that his cancer was at an advanced stage. Cofidis effectively pushed him out by offering a contract whereby he would have been paid by the kilometer.
Armstrong added: "The best thing for David really would be for him to go look elsewhere. To change his environment, his methods, and his discipline. Right now I can't say whether he'll come to our team.
"He's expensive and we can't overspend our budget. All I can say is that he would be a huge boost to the team because he's good at time trialling and he's not too bad in the mountains either.
"But he must learn to take things in moderation. David's a bit like a rock star. He's a bit mad, but in a nice and brilliant way."
Millar, who has a reputation for partying and generally living life to the full, told AFP at the team's presentation in Paris three weeks ago he had once more been given "carte blanche" by Cofidis in this year's races.
His team manager Alain Deloeuil immediately countered by saying the team were expecting more from the 26-year-old Biarritz native.
"David was a bit compromised last year because he was ill at the start of the season, but this season we expect him to start working harder than he's been used to," Deloeuil told AFP after the team presentation.
"He has a lot of class, and although he's still young we expect him to come to the fore this year. He can and should be winning bigger stage races."
Elsewhere, Guimard, who since giving up being a successful coach has been a radio consultant for major races, will decide at the end of the week whether or not to take up a job offer from second division French outfit BigMat Auber, who took part in the Tour in 2001.