TAMPA -- Torey Krug will not play for the Boston Bruins for the remainder of the Eastern Conference Second Round against the Tampa Bay Lightning, starting with Game 5 at Amalie Arena on Sunday (3 p.m. ET; NBC, CBC, TVAS) because of a left-ankle injury.

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Nick Holden will replace the defenseman in the Bruins lineup. Boston trails the best-of-7 series 3-1.Tampa Bay would advance to the Eastern Conference Final with a win. Game 6, if necessary, is Tuesday, with Game 7 scheduled for Thursday.

Krug reportedly was seen leaving TD Garden using crutches and with his foot in a walking boot following the Bruins' 4-3 overtime loss in Game 4 on Friday. He was injured when he went ankle-first into the boards with Lightning forward Alex Killorn behind him at 8:30 of the third period.

Krug is fourth on the Bruins with 12 points (three goals, nine assists), including seven assists on the power play.

"It's just tough, tough luck, especially a guy like that," Bruins center David Krejci said. "He's been having a really good season and good playoffs. You can't replace a guy like that, but at the same time we've been missing lots of key players throughout the season and guys have been stepping up. It's not just up to the [defensemen] to step up, but also the forwards to help out defensively on breakouts, get open, just play a little better in every single position."

Holden was a healthy scratch for the past eight games. He last played in Game 3 of the first round against the Toronto Maple Leafs, when he had an assist in 12:25 of ice time.

The Bruins acquired Holden in a trade from the New York Rangers on Feb. 20. He had five points (one goal, four assists) in 18 games with Boston in the regular season.

"He did a good job for us when he went in," Cassidy said. "We're going to miss Torey, obviously. He's a good player for us and really adds to our offensive side of things, but Nick's done a good job so in he'll go."

Cassidy said he'll use Holden on the third defense pair with Adam McQuaid and move Matt Grzelcyk up to play in Krug's spot on the second pair with Kevan Miller. Zdeno Chara and Charlie McAvoy remain the top pair.

Cassidy wasn't as certain about the forwards he'll use in Game 5. Ryan Donato and Brian Gionta played for the first time in the series in Game 4 with Danton Heinen and Riley Nash scratched. Tim Schaller also went back in for Tommy Wingels, who replaced him in Game 4.

"We'll have to look at how it played out last night, the combinations we used and players that didn't play, is our best lineup with them back in?" Cassidy said. "I don't have a good answer for you today, but we'll sort through that."