While Cook's on-screen appearances soothe investors in the short term, the positive effect doesn't stay long term, Toni Sacconaghi, senior analyst at Sanford C. Bernstein, told CNBC's "Squawk Alley."
"The old adage of 'Do what they do, not what they say,' holds true in this case with Apple," Sacconaghi said.
Since becoming chief executive, Cook has made seven major on-screen appearances, Sacconaghi wrote in a research note. Three months after each one, Apple underperformed the S&P 500, despite bouncing the day after the appearance in all but one case, Sacconaghi wrote.
Cook, usually tight-lipped about Apple's strategy, appeared on CNBC's "Mad Money" on May 2. The tech giant's shares are down nearly 1 percent month to date, while the S&P 500 is up modestly, according to FactSet.
On the show, Cook told Jim Cramer that if people switch from Android to Apple phones, "the market doesn't need to be growing hugely for Apple to grow." Cook's comments came after the company reported its first year-over-year quarterly sales drop since 2003.
Apple last month reported fiscal second-quarter earnings of $1.90 per diluted share on $50.56 billion in revenue, below the $2 a share on $51.97 billion in revenue projected in a consensus estimate from Thomson Reuters.
Sacconaghi has long said the smartphone market might soon become too mature to support aggressive growth: He told CNBC last year he believed Apple's best days were behind it.
"Last year Tim Cook was asked about whether the iPhone replacement cycle was accelerating — whether the iPhone 6 was really pulling forward users — and he said 'Not really,'" Sacconaghi said. "Then on this earnings call they did talk about the fact that last year there did seem to be an acceleration ... so there are some incongruities that we observed."
To be sure, it would be tough to beat Apple's boom year last year, Sacconaghi said, even as the company reportedly prepares to launch the iPhone 7. While Sacconaghi is worried the next iPhone won't be "groundbreaking" enough to revive Apple, he did have a tip for when the company's shares tend to move higher.
"What we did find to be a more helpful indicator is when Apple really accelerates its share repurchases," Sacconaghi said. "When that amount was doubled the company significantly outperformed for the next four to seven months to the tune of 20 percentage points relative to the market."
— CNBC's Everett Rosenfeld and Abigail Stevenson contributed to this report.