Redskin cornerback Pat Fischer is to undergo surgery today at George town University Hospital to relieve pain from a pinched nerve in his lower back.

Dr. Alfred J. Juessenhop, professor and chairman of the department of neurosurgery at Georgetown, said tests completed yesterday showed Fischer was suffering from the same ailment for which he underwent surgery in 1972.

The only difference this time. Luessenhop said, is that it affects Fischer's left side instead of the right side. The latest injury, first suffered while the 17-year NFL veteran was lifting weights in training camp, has recently made it difficult for Fischer to even walk without pain.

Luessenhop said the pinched nerve was caused by a partially ruptured disc and by slippage in Fischer's vertebrae. Fischer has suffered from the slippage throughout his life and it is the major cause of his pain, the neurosurgeon said.

"He doesn't have much space for the nerve to pass through," Luessenhop said, " so the nerve gets entrapped by a minor herniation of the disc. We will enlarge the opening to the spine. It may involve removing some bone or some disc."

The doctor said he foresees no problems for the feisty Fisher once he recuperates.

"He should do okay," Luessenhop said. "We did the other side, and he played football for five years after that."

Fischer, who was placed on the injured-reserve list Monday, said he was happy the origin of his pain had been discovered and that he could take the ordeal of another major operation because "it's a matter of being able to get some relief."