The British squad endured a tough outing at Rally Estonia last weekend with French youngster Adrien Fourmaux finishing as its highest-placed driver in seventh.
Lead driver Craig Breen notched up a handful of top-three stage times but was out of the battle on Friday morning when he damaged the suspension after hitting an anti-cut block. Pierre-Louis Loubet and Gus Greensmith both retired on the final day.
Sébastien Loeb brought M-Sport its only victory of the hybrid era at Rallye Monte-Carlo in January, but the nine-time world champion is contesting only a small number of selected rounds.
Millener believes the addition of an established driver to support Breen on a regular basis would open doors to future successes.
“It’s always a negative when your lead driver retires or has a problem,” he told WRC.com. “When that happens, we often find ourselves struggling to challenge for the top three. It’s frustrating, but it’s understandable.
“I think it’d be fantastic to have another established driver to be able to push for those podiums, but unfortunately it’s just an ambition we can’t reach at the moment.
“We’ll keep coming up with ideas of how we can change that, because we know we can get the results. We’ve just got to keep on trying and hoping that we can find a way to get more funding to do that.
“Craig is quite capable of doing it, but as the only driver leading the team he has a lot of pressure on his shoulders. I don’t see any way at the moment for us to find the budget to get that additional driver, but it's something we should always aim for,” he added.
While Millener was quick to praise youngsters Fourmaux, Greensmith and Loubet, the team currently sits third in the manufacturers’ championship standings and he admitted a solution needed to be found sooner rather than later.
“Adrien, Pierre and Gus are all still learning,” he explained. “Of course, in the longer term, we’ve got three really good young drivers and hopefully in three or four years’ time, they’ll be the ones filling that position. But we also need to do it right now as well.
“We can’t give up - if you do that and just live with what you have, you’re never going to make it work. It's just an ongoing thought in my head that we need to find a way to try and do it. It’s not easy, but I won’t give up.”