As pressure mounts on McDonald's to add a vegan burger to its lineup, CEO Steve Easterbrook says consumers should "watch this space."

He likened the decision to add a meat substitute to the menu to its decision to start offering breakfast all day.

"The question is: Will the demand make absorbing it worth the complexity because it will drive the business? We had a similar discussion maybe four years ago about All-Day Breakfast," Easterbrook said on CNBC's "Squawk on the Street."

After being implemented, All-Day Breakfast spurred same-store sales growth for several quarters. But competitors took notice and beefed up their breakfast offerings. Earlier this month, amid its broader push to speed up service, McDonald's said franchisees could trim their All-Day Breakfast offerings.

McDonald's has been under pressure in recent months to add a vegetarian-friendly burger to its menu as more and more of its competition has taken the plunge.

"I don't think it's faddish," he said. "Whether it maintains the same level of buzz is what's interesting."

Its German locations sell a plant-based burger made by food giant Nestle, but McDonald's has said that individual markets decide what is best for their customers.

The plant-based craze also arrives as the Chicago-based company has been trimming its menu to reduce complexity at its restaurants. It has scaled back its late-night offerings and nixed its line of premium burgers.

"When you look at the whole meat-substitute type ideas, I think what will be interesting for us will be to see who is particularly interested in that," Easterbrook said to CNBC's Carl Quintanilla on Wednesday. "Is it an existing customer who just wants an alternative option; does it bring a new customer in?"