We still don’t know a lot about Braden Holtby, but Bruins fans now know enough to worry. For more than 60 minutes Thursday night, Washington’s rookie netminder, previously known as merely the last resort on the Capitals’ depth chart, came within a glove save of handing the Bruins a Game 1 upset in this Eastern Conference quarterfinal series. “I don’t really know what happened,’’ lamented the fresh-faced 22-year-old, moments after Chris Kelly’s pinpoint slapper off left wing rocketed by his glove hand (left) only 1:18 into overtime to give the Bruins a 1-0 victory at the Garden. “I kind of lost it a little bit.’’ Over the course of the night, though, the playoffs found someone who could be a hero in incubation. Time and again, Holtby turned back the best the Bruins could fire his way, including 17 shots in the second period when a rash of penalties kept play mostly at the Washington end of the rink and pressure jacked up on Holtby. By the end of 60 minutes, Who-the-Heck-is-Holtby? had 29 saves, and still owned a shutout, matching the goose egg shaped by the goalie at the other end of the ice, Tim Thomas, who just happened to be last year’s Conn Smythe winner as the MVP of the playoffs. Fans of a certain age were chirping in the stands and tweeting about Holtby resembling Ken Dryden, who in 1971 was the relatively unknown Canadiens goalie who snuffed out the Bruins in first round. The ’71 Bruins were expected to romp to a second Stanley Cup title in as many years. These Bruins, you might recall, won the Cup only last June. The young Dryden, not long removed from the Cornell campus, sealed off the Habs net and shut down the game’s most powerful offensive engine, canceling the Cup parade and party plans at Government Center. “That big octopus!’’ bemoaned Phil Esposito, his Bruins sent packing after losing Game 7. “We just couldn’t beat him.’’ Esposito collected 76 goals and 76 assists for a career-high 152 points that season, but was kept fairly quiet with a line of 3-7-10 in seven games against Montreal. When the series was over, and the Bruins were on their way home, they knew who Dryden was, but they knew precious little about how to beat him. At the moment, that’s right where these Bruins are with Holtby. They had the Kelly slapper to thank for the W, not to mention a key save by Thomas on Marcus Johansson approximately six seconds earlier, but they’re going to have to dig deeper, find ways to put pucks behind the rookie. “We only beat him once, so we can’t say we’ve got a book on him,’’ said Boston winger Brian Rolston, who picked up the second assist on Kelly’s winner. “Heck, if we had the book, we’d have had eight goals. He’s a good goalie, and he’ll continue to be a good goalie.’’ If Game 1 was one to grow on, the 6-foot-1-inch Holtby had to add a few inches of confidence in his playoff debut. He turned back Boston’s top point-getter, Tyler Seguin, on five shots. Rich Peverley went 0 for 4. Rolston, Joe Corvo, Daniel Paille, and Patrice Bergeron all landed three shots, all came up emptier than the Zamboni’s water tank on July 4. Kelly, sprung over the blue line by a Benoit Pouliot feed, raced to the top of the left wing faceoff circle with ex-Bruin blue liner Dennis Wideman in pursuit. More a playmaker and defense-conscious pivot, Kelly pulled back and took aim as the alert Holtby moved out slightly toward the wing and hunkered down low. Kelly’s shot ticked ever-so-slightly off of Wideman’s stick, just enough that Holtby appeared to lose the angle. It whizzed by his glove and caught the net to the far side, just inside the post. “I was probably the most surprised guy in the building,’’ said Kelly, noting that he couldn’t have expected to be successful with a clear slapper, not in today’s hockey, with goalies plucking long slappers as if they are low-hanging fruit. “The goalies are just so good. The days of going down the wing and beating a goalie with a slap shot are long past.’’ Long gone, faded into oblivion with the likes of Bobby Hull, Boom Boom Geoffrion, Guy Lafleur, and Brett Hull. Big goalies. Big pads. Precious little room to score. But Kelly connected, though it took the delicate deflection of Wideman’s stick to provide the margin of error. “He’ll be fine,’’ said coach Dale Hunter, a man of economic words, sizing up Holtby’s night. “He has lots of confidence. It was one of those shots. Not too many goalies would have had it.’’ Thomas might have had it. Though he didn’t face many shots, 17 in all, he was superb when it counted most. He turned back Johansson’s shot with a pad save, triggering the long transition play (Joe Corvo-Rolston-Pouliot-Kelly) that closed the show. Who The Heck is Holtby? Born in Lloydminster, Saskatchewan, home of the late Bruins forward Ace Bailey, he was the Saskatoon Blades’ workhorse for three years in the Western juniors before turning pro in 2009. The Capitals drafted him with the No. 93 pick in 2008. For the past two years, he’s been called up from Hershey to fill in for injured Capitals. If not for injuries to Tomas Vokoun and Michael Neuvirth right now, most regular NHL watchers still wouldn’t know his name. After what they saw of Holtby in Game 1, it’s a good bet the Bruins would prefer to forget him.