CRANBERRY, Pa. -- Pittsburgh Penguins center Sidney Crosby has been diagnosed with a concussion and will miss Game 4 of the Eastern Conference Second Round against the Washington Capitals on Wednesday, coach Mike Sullivan said Tuesday.

"He will go through the protocols that we always put our guys through when they've been diagnosed with a concussion," Sullivan said. "You know, the nature of these things is they're all very different. Sometimes they come around quickly. Other times they don't. My experience of dealing with these in the past with players is that they're day-to-day things.

"So we rely on our medical staff to advise us in the right way, and our guys do a great job in that regard."

Crosby was injured when he was cross-checked in the head by Capitals defenseman Matt Niskanen 5:24 into the first period of a 3-2 overtime loss in Game 3 on Monday.

After remaining down for a few minutes, Crosby was helped to his feet by teammates Ron Hainsey and Patric Hornqvist and glided off the ice hunched over before heading to the locker room.

Crosby's left knee appeared to buckle under him after initial contact from Alex Ovechkin that caused him to begin falling before Niskanen hit him, but Sullivan confirmed there was no injury to Crosby's lower body.

Video: Sullivan announces Crosby out with a concussion

Niskanen received a five-minute major for cross-checking and a game misconduct.

The Penguins, who lead the best-of-7 series 2-1 with Game 4 att PPG Paints Arena (7:30 p.m. ET; NBCSN, CBC, TVA Sports), will also be without right wing Conor Sheary for Game 4 after he sustained a concussion in a collision with Hornqvist in the second period of Game 3.

Sullivan said Sheary skated on his own before an optional practice Tuesday as part of the concussion protocol and will continue to be evaluated. Crosby did not skate but was at UPMC Lemieux Sports Complex on Tuesday, and Sullivan said he was in good spirits.

"He's very upbeat and he's very positive," Sullivan said. "We're very optimistic, and we're hopeful that we'll get him back in a timely fashion."

Seeing Crosby seemed to alleviate some of his teammates' concerns.

"He seems good," goaltender Marc-Andre Fleury said. "He was very encouraging towards us for [the Penguins' play in Game 3]."

The Penguins were already missing defenseman Kris Letang following season-ending neck surgery and goaltender Matt Murray, who sustained a lower-body injury in warmups before Game 1 of their first-round series against the Columbus Blue Jackets.

But losing Crosby, who won the Conn Smythe Trophy when the Penguins won the Stanley Cup last season and is a finalist for the Hart Trophy this season, is their biggest blow. He has 11 points (four goals, seven assists) in eight Stanley Cup Playoff games.

"Definitely that's a tough one," Fleury said. "He's obviously our best player. He's our captain, he's our leader. You can't replace him, but I think we've shown over the season that whenever a good guy is not there that the other guys have to step up, and we're lucky to have some depth on our team and I think we have enough to win games."

Because of Crosby's history of concussions, it is always a concern when he is diagnosed with another one. He missed the first six games of this season with a concussion he sustained during a preseason practice. He also missed 101 regular-season games between the 2010-11 and 2011-12 seasons because of concussion-related issues.

Crosby experienced concussion symptoms after taking hits in consecutive games Jan. 1, 2011 against the Capitals and Jan. 5, 2011 against the Tampa Bay Lightning, and missed the remainder of the 2010-11 season, the 2011 Stanley Cup Playoffs and the first 20 games of 2011-12. He returned to the lineup Nov. 21, 2011, but played eight games before a recurrence of concussion symptoms sidelined him until March 15, 2012.

Video: WSH@PIT, Gm3: Crosby exits with injury early in 1st

So seeing Crosby down on the ice after being hit by Niskanen was difficult.

"That's a tough moment for sure," Fleury said. "I care a lot about him, about my teammates, and we've been friends for a long time. I know he's always very devoted to the team and always working out there. So we just hope he feels better soon and see him smile again."

Fleury, the only Penguins player remaining from before Crosby's rookie season of 2005-06, recalled how Crosby supported him when had a concussion that caused him to miss the start of the playoffs last season.

"He was great," Fleury said. "He's been through it, so I think he knows a lot [about] what goes through your mind at that point and he was great at helping me out and relaxing me, just getting through. He's a good friend. That was nice of him."