Albert V. Casey, who assumed the job of postmaster general last week with a wisecrack that he would stay "if I'm really good, six months," will become a business professor at Southern Methodist University this fall.

The announcement came as no surprise to the U.S. Postal Service Board of Governors, who knew before they ousted Paul N. Carlin and hired Casey that they would soon be looking for Casey's successor.

"We were aware from the outset that he couldn't stay beyond September," said Peter Voss, board vice chairman. "We made it very clear [in announcing the appointment] that we were proceeding with the search for Mr. Casey's replacement."

The choice of Casey, a former president of American Airlines and the Times-Mirror Co. in Los Angeles, was widely interpreted as a move to bring corporate-style management to the Postal Service and strengthen the hand of the board of governors.

Voss said it is not unusual in the business world to bring in a "finance man" to take charge during a major corporate restructuring.

"In a sense, there's a parallel here," he said. "If we can get nine months out of Mr. Casey and they are good strong months, it will equal whatever we could get out of someone who is in a lot longer."

Carlin, who was fired after barely 12 months, was criticized for not moving fast enough to streamline the agency's bureaucracy and improve its financial position.

"We ran into a replacement for Mr. Carlin earlier than we expected," Voss said. "When we ran into the possibility of Casey, we thought we should take advantage of that right now. We felt that decisions weren't being made that should be made."

At the Dallas university, Casey, 65, will become the first holder of the Ann Cox Distinguished Professorship in Business Policy.

Casey did not return a telephone call yesterday, but in a statement released by SMU he said the academic position "presents a great opportunity for me to close out a business career with great satisfaction."

SMU spokeswoman Ann Abbas said Casey had committed himself last year to fill the newly endowed chair. The appointment was not announced until yesterday because "we hadn't cleared all the red tape," she said. "I guess he's planning to do a quick job in Washington."