It is a fine and precise procedure to reconnect a detached retina to the main wall of the eye, one that involves freezing parts of the detached tissue at temperatures as low as 80 degrees below zero centigrade, then patching it to the eye's wall with a silicone substance.

On Sunday, Dr. Ronald G. Michels, an associate professor of ophthalmology at Johns Hopkins University Hospital's Wilmer Eye Institute, and two associates apparently used that procedure to connect a detached retina in the left eye of world welterweight boxing champion Sugar Ray Leonard in a two-hour operation that nonetheless left Leonard's boxing career in jeopardy.

Leonard was reported in good condition and making normal progress toward recovery yesterday in the Baltimore facility. A spokesman said no complications had developed, although it may be weeks before doctors are able to determine whether there will be a complete recovery. It likely will be months before there is a decision on whether or not Leonard will resume boxing.

One of the primary instruments of vision, the retina is the sensory membrane at the back of the eye that receives light images and transmits them to the brain through the optic nerve. Sight is a function of the brain, but without the retina attached to the eye it is impossible, Michels said.

In Leonard's case, just under 50 percent of the retina had separated from the eye, according to Michels. Although there is no way of being certain what caused the retina to separate, Michels acknowledged a blow to the eye could have been the cause.

There are several procedures to correct such a condition. At Hopkins, the procedure involves first attempting to locate the area of detachment and any holes or tears in the retina. Doctors probe the back of the eye with a delicate instrument while peering through the front with an ophthalmoscope.

Retinal detachment occurs when the retina becomes torn or pierced, allowing fluid from the eye to seep through, peeling the retina away from the inside of the back of the eye, according to Dr. Leonard Parver, director of the eye trauma unit at Georgetown University Hospital. In many respects the detachment is not unlike a large blister.

Once located, the breaks are closed through a process called cryotherapy, which involves freezing the retina around the hole with an instrument called a cryoprobe. The cryoprobe has been treated with liquid nitrogen to bring it to temperatures ranging between 60 and 80 degrees below zero centigrade. The freezing will act later to hold the retina in place, once the physician repositions it.

Then the detached part of the retina is treated with a silicone substance, an organic compound used in adhesives and lubricants and frequently in plastic surgery. If the retinal surgery works, the silicone substance covers the blister-like separation, pressing it back against the wall of the eye.

In Leonard's case, according to his lawyer Mike Trainer, the prognosis for a full recovery is good since the tear in the retina was away from the center of the eye and there was no interference with vision. Still, Leonard did see some spots in his left eye's field of vision while training in Buffalo for a title defense Friday against Roger Stafford. That fight has been postponed indefinitely.

Trainer said yesterday the injury shouldn't necessarily halt Leonard's boxing career if recovery progresses properly. "We have every reason to believe the eye will be more sound after the operation," he said. " . . . Dr. Michels said it was almost impossible that the retina would tear in the same place as before."

But Trainer also stressed at length that no one would try to pressure Leonard to return to the ring. "On fighting, no one will talk to Ray about it. If he never mentions it, we will never mention it."

Dr. Bernhart Schwartz, a ringside physician at boxing matches in Los Angeles, told the Los Angeles Herald Examiner yesterday that Leonard would be foolish to fight again.

"He would be in real danger of going blind in that eye," he said. "Never would I give him the okay to fight. If he decided to fight, the retina could be detached again and he'd be sunk."

At Johns Hopkins yesterday, Leonard spent most of the day sleeping, and only immediate family members were allowed to visit. Hundreds of well-wishers called and sent telegrams, according to a hospital spokesman, including President Reagan, who called from Chicago.

According to a White House statement, the President conveyed "his good wishes as Mr. Leonard recovers from surgery," and "his hope that Mr. Leonard's career will continue."

Leonard, according to the White House, "expressed his appreciation to the President for his call and concern."