In 2007, his thoughts turned again to tickets. The explosion of Internet sales and sites got him thinking. He joined AudienceView, a Toronto ticketing company, but left after eight months. Then, last year, after sensing the discontent over Ticketmaster’s merger with Live Nation, he approached Jean-Francoys Brousseau, a former chief technology officer at Ticketmaster who had started Outbox.

“There was nobody out there that knew the business as much as he did,” Mr. Brousseau said of Mr. Rosen. Outbox had already signed up Cirque du Soleil, and, with Mr. Rosen onboard, managed to sign up AEG as a partner as well.

When the Justice Department approved the Live Nation merger, it stipulated that AEG would be allowed to use Ticketmaster’s technology for five years, at which point it must switch to an independent platform. This was intended to ensure that competition remained in the ticketing industry. Outbox gives Mr. Leiweke and AEG a head start.

“I’m cheering Fred on mightily,” Mr. Leiweke said.

Many of the venues, however, are locked into long-term contracts with Live Nation — five years, on average, according to Benjamin Mogil, a director at the Stifel, Nicolaus brokerage firm.

Mr. Rosen said that about 20 percent of those contracts, which represent more than 100 venues, come up for renewal annually. He said new approaches to ticket pricing — like selling tickets, parking and food as a package — and flex pricing could tip the scales. He is also in talks with phone companies to put mobile ticket purchases on a person’s phone bill, which would make it easier for people to buy tickets using cellphones.

Even Mr. Diller, who resigned as chairman of Live Nation last year after a boardroom struggle, isn’t ruling out Mr. Rosen’s new venture.

“It’s what he knows, and he knows it awfully well,” Mr. Diller said.

Mr. Rosen, for his part, just seems happy to be back in the ticket game. “I always prefer to be the underdog,” he said. “As the underdog, no one expects anything.”