The Bruins are now in must-win territory — perhaps earlier than they expected to be.

Despite a statement victory in Game 1, the Black and Gold have now lost three straight games to the Tampa Bay Lightning after a 4-3 overtime loss in Game 4. A fourth defeat will send them home for the summer, ending a season of unexpected success that elevated once-tempered expectations to levels of Cup or bust.

This all happened very fast.

If we’re being honest, the series feels slightly closer than a 3-1 Tampa Bay lead would indicate. Obviously, the Lightning have held the edge. That’s not a fact anyone is disputing. But aside from Game 1 — in which the Bruins totally outmatched the Lightning — and Game 3, in which Tampa returned the favor, momentum has shifted on a period by period basis.

Yet something hasn’t been clicking for Boston — enough for head coach Bruce Cassidy to tinker with the team’s lineup for the first time in this series heading into Game 4. Ryan Donato and Brian Gionta were inserted into the bottom-six, in lieu of Danton Heinen and Riley Nash, with hopes of creating a spark that was missing the past two games.

The change didn’t work.

Donato and Gionta flanked center Sean Kuraly on either side, creating a hodge-podge of a line that featured three players with three very different skill sets. By the time the third period came around, Kuraly was bumped down (or maybe up?) to center David Backes and Tim Schaller — making for a slightly more congruent line — while Noel Acciari filled the hole with Donato and Gionta. Even then, the latter trio still seemed like a mismatch.

Guess which line was on the ice for Dan Girardi’s game-winning-goal?

The move to play Gionta — who last played in a game on April 8 — over Heinen, Riley Nash, and Tommy Wingels was truly one of Cassidy’s most questionable decisions all season. For a team that so badly needed to be faster coming out of Game 3, it’s hard to understand what made the 39-year old winger an appealing option. Now, for the must-win Game 5 on Sunday, Cassidy is going to have to alter his lineup again.

Assume that Torey Krug, who left TD Garden on crutches and in a walking boot, is out. Then in comes Nick Holden, the new addition to a defense that probably doesn’t need much tweaking aside from in-game match-up adjustments.

But what about the Bruins’ offense? Cassidy was adamant about sticking with the guys who got them to the second round via a Game 7 win against the Toronto Maple Leafs. But now? Time to go with the lineup that will give you the best chance to win, regardless of anything else.

So, who’s in?

Let’s start at the top and say that the Bruins’ top line of Brad Marchand, Patrice Bergeron, and David Pastrnak stay where they are — pending an impressive attempt from our proud hockey purists up north to suspend number 63 for (gasp) licking an opponent. But that’s a column for another time.

David Krejci is in, though it’s well past time for “playoff David Krejci” to make an appearance. Rick Nash and Jake DeBrusk are both in, too, but it might be best for one of them to finally add some energy to a drowsy third line. Perhaps DeBrusk, who has been consistently great all postseason, can add new life. After those six, it’s not a stretch to think that any player in the bottom half could be a healthy scratch.

David Backes, especially, presents an interesting case. The veteran started out on the fourth line with Schaller and Acciari in Game 3 and finished playing only a paltry 9:27 at even-strength — second lowest of any Boston player and the fewest minutes he’s played in a game all season (minus ejection or injury). He could certainly be a candidate for a healthy scratch, though it’d be surprising if Cassidy didn’t dress Backes in a must-win game where, if at his best, he could make his presence felt.

Riley Nash should draw back in, but at whose expense? Perhaps Acciari, who played a team-low 9:12 and hasn’t looked himself at all in this series. While we’re at it, Gionta seems like a pretty safe bet to not see the lineup after Girardi pushed him away from the puck before netting the game-winner in Game 4.

Next, since Tampa will have last change on home ice, it’d be wise to err on the side of defensively-minded forwards for optimal match-ups. Add to that the way the games have been officiated so far in this series, and penalty killers become preferable. Thus, insert Schaller and Kuraly.

If you’re paying attention, you’ll remember there’s an empty spot on this hypothetical second line. Put Donato there. He wasn’t going to produce with Kuraly and Gionta. If he can’t do so with Krejci and Rick Nash, then, well, see you next year. But Game 5 is a must-win, and that means going for it.

Flip a coin for the last spot. Wingels can draw penalties, play shorthanded, and create turnovers, while Heinen has an adjustable skill set that could fit on any of the four lines when at even strength. The Bruins haven’t scored an even-strength goal since Krug’s third period tally in Game 2 — a drought of 127 minutes and 22 seconds. The last time the Bruins needed a big goal? Heinen.

So, maybe these are the Black and Gold lines for Game 5:

Marchand-Bergeron-Pastrnak
Donato-Krejci-Rick Nash
DeBrusk-Riley Nash-Backes
Schaller-Kuraly-Heinen

Or, maybe not.