Having previously insisted that the striker would be going nowhere this summer, Aulas admits interest from other clubs means he fears he can no longer put off the inevitable.
Despite Benzema saying he wants to stay at Lyon at least until the end of next season, Aulas is aware that money is likely to do the talking.
"Karim has said he wants to stay at Lyon but I think Cristiano Ronaldo and Kaka also imagined they would be staying with their clubs," he told L'Equipe.
"There is a tsunami known as Real Madrid sweeping through football which leads to a reaction from Chelsea, City and tomorrow United.
"I won't hide the fact he has been approached by other teams - as a consequence, saying that he is non-transferable is tough."
Sir Alex Ferguson and Manchester United are said to be the front runners to land Benzema who has scored 54 goals for Lyon in two seasons.
With £80 million soon set to be injected into the Scot's transfer kitty from the sale of Cristiano Ronaldo, Ferguson will have no problems matching Lyon's asking price, thought to be around 40 million euros (£35.2m).
Meanwhile, United have failed in their attempts to sign Barcelona goalkeeper Victor Valdes who, after an extended period of stalling, has agreed to a four-year contract extension keeping him at the Nou Camp until 2014.
With his original contract due to expire next season, talks between player and club broke down over the issue of wage demands.
Even if United had tabled an offer for the 27-year-old, it is unlikely they would have agreed to a deal any where near his current weekly salary of £150,000-a-week.