Chrysler Corp. yesterday named Robert S. Miller Jr. to take over as the automaker's vice chairman, the No. 2 spot behind Chrysler Chairman Lee A. Iacocca.

The position opened up Wednesday with the surprise announcement that Gerald Greenwald, Iacocca's heir-apparent, was leaving to head a union effort to take over UAL Corp., the parent of United Airlines.

Greenwald's departure, along with those of two other top executives in the past month, have raised concerns on Wall Street and within the company that Chrysler was facing a potential leadership crisis at a time when its car sales and market share were dropping dramatically. But one of those departing officers, Vice President and Treasurer Frederick W. Zuckerman, discounted such concerns.

"You have three people leaving for their own personal reasons," Zuckerman said.

"One has nothing to do with the other. That all three are leaving near the same time is nothing more than an unusual confluence of events," said Zuckerman, who helped to rescue Chrysler from bankruptcy in 1979 and 1980.

As for his own exit, Zuckerman said: "There is no one guy, there are no two, three or four guys with whom Chrysler can't do without."

Michael Hammes, Chrysler's vice president for international operations, resigned last month to take a job as vice president of Black & Decker Corp. and president of the Hunt Valley, Md., firm's power tools division.

Miller, 48, is seen by company insiders as a worthy Greenwald successor. But few people at the company or in the auto industry view him as Iacocca's successor. Without Greenwald, sources said, there is reason to believe that Iacocca, 66, will hold onto the chairmanship beyond 1991, when he has said he would retire.

"Chrysler is in trouble right now, and Lee does not want to leave as a loser," said another Chrysler official, who asked not to be named. The official pointed out that Iacocca has launched a $15 billion program to bring out new Chrysler products and that he has also instituted a program to trim $1 billion annually from the company's operating costs.

"It's going to take him at least until 1993 to do what he wants," the Chrysler official said of Iacocca.

Zuckerman offered no speculation on how long Iacocca will remain in place. However, for the moment, he said, "Lee is firmly managing Chrysler. He is still firmly in control."

Miller, who now is the chief financial officer, will assume responsibility for some of the jobs previously handled by Greenwald, including oversight of Chrysler Financial Corp., Chrysler Technologies and the company's international operations. And though he has the title of vice chairman, Miller will not share a two-seat "office of the chairman," as did Greenwald. That office has been abolished. The chairman is Iacocca.