John Davidson, the New York Rangers' goalie, missed 18 games late in the regular NHL season with a sciatic nerve condition in his leg. When he returned April 1 and yielded six goals in a 7-3 loss to the Philadelphia Flyers, it seemed unlikely that he would be a factor in the Stanley Cup playoffs.

Coach Fred Shero, however, stuck with Davidson, despite the presence of veteran Wayne Thomas and sharp young Doug Soetaert. Once again, Shero has been proven correct.

In five playoff games, Davidson has compiled a 1.37 goals against average, winning four games in which he yeilded one goal each time. He has been a key factor in the Rangers' building a 2-1 quarterfinal lead over the Flyers, with game four in New York tonight.

Friday, as Davidson stopped 32 Philadeaphia shots in a 5-1 victory, the Madison Square Garden fans resumed their occasional salute of "J.D., J.D." in thunderous unison. The 6-foot-3, mustachioed goalie was appreciative, but he wasn't letting it go to his head.

"I seem to be finding the puck and our guys are creating situations where the Flyers don't get good shots," Davidson said. "There are very few three on ones or two on ones. We used to give up five or six of them a period and then people would say the goal tending was terrible."

As Washington Capitals fans can attest, Davidson is capable of some terrible nights. He was in the nets for a 5-1 loss at Capital Centre and he was pathetic in a 6-6 tie at Madison Square Garden. But when he's good, he can be very, very good.

"John's the kind of guy you want to be in goal for you," Shero said. "He's a good guy, a good leather with a lot of good qualities. He's strong as a bull to start with and he seems more confident. About his only weakness has been missing the puck behind the net."

As long as Davidson doesn't miss it in front, neither Shero nor those rowdy Ranger fans will be complaining. The spectators can be brutal, however, whether spitting on the visiting Flyers, as they were when Friday's game ended, or giving Davidson the razzberry, as they did when he helped surrender a three-goal lead to the Flyers in a disastrous third period a few weeks ago.

"We've probably blown more third period leads than any other team," Davidson said. "But we're learning how to win. The Flyers have been there before, but we haven't. We're getting more confident, though.

"Going into the series with Los Angeles, we didn't know how well the team would play, or how well I'd play. It's a credit not only to myself, but to 18 other guys that we're playing with intensity and getting the job done.

"The Flyers are physical and they play well, but we honestly believe we're as good and as strong as they are."

Davidson was a first-room draft choice of the St. Louis Blues in 1973, a rare distinction for a goaltender. He came to the Rangers in 1975 in a big trade for forwards Ted Irvine, Jerry Butler and Bert Wilson.

Although he compiled an excellent 3.18 goals-against record a year ago, Davidson had played only three Stanley Cup games until last week. He has been in five more now, and if he continues to make miracle saves, the Rangers figure not only to knock out the Flyers, but also to give the New York Islanders a battle in the semifinals.

"I try not to get overly excited," Davidson said. "I've been playing in pressure-filled games for 10 or so years, so this isn't really something new. When I was a junior (in Calgary), the junior games were really big in Western Canada and we had 6,000 or 7,000 people. There was plenty of pressure then."

Anybody who can relate the pressure of a Stanley Cup battle in front of 17,000 fanatics to a junior playoff game has won his personal psychological war.

Don Maloney, the Rangers' offensive hero with four goals in three games, says he has had trouble sleeping. If Davidson has had any difficulties at all, they are not apparent. And the Rangers have overcome their No. 1 problem by ending that 10-game home-ice winless string against the Flyers.

Chicago, Pittsburgh and Toronto, all of whom have been having problems, are the hosts in the other quarterfinal playoffs tonight. Chicago faces the Islanders, Pittsentertains Boston, and Toronto is at home to Montreal. CAPTION: Picture, Penguin goalie Denis Herron can't quite get a puck and Peter McNab of Bruins get himself a goal as John Wensink watches.