ATLANTA — There’s just one critical question for Atlanta United ahead of Thursday’s second-round playoff match: How do they fill the hole in defense left first by Miles Robinson, and then Michael Parkhurst, and how will that decision impact how they play?

Robinson, one of the best defenders in MLS and a linchpin in United manager Frank de Boer’s strategy, hurt his hamstring more than a week ago while training with the U.S. men’s national team (De Boer was not pleased.)

RELATED: Atlanta United's Frank de Boer frustrated over Miles Robinson's injury ahead of important playoff game

Then Parkhurst, the 35-year-old team captain who seems to play like he’s 25 in the playoffs, also got hurt after stepping into – and flourishing in - Robinson’s role last weekend.

Now De Boer has to decide if he believes anybody else on the team can fill the role or switch up how they play entirely.

Typically, Atlanta play with three defenders at the back – two on the wings who are given free reign to push up and, in turn, aggressively help advance the rest of the attack, and one in the middle who stands as the last line of defense between the opposing team’s attacking players and goalie Brad Guzan if the situation turns.

That man in the middle is usually Robinson, and his speed, strength and intelligence make him exceptional at providing cover for the rest of the team to push forward. Last week, Parkhurst did an excellent job at it, too.

Atlanta United vs. Philadelphia Union

  • Tickets: Still available for as low as $50 on Ticketmaster
  • Time: Thursday, 8 p.m.
  • Venue: Mercedes-Benz Stadium
  • TV: ESPN2

The alternative is to play with four defenders at the back – in theory a bit safer, but also less dynamic as it removes an attacking player from the lineup. De Boer actually did this in the second half of the last game, subbing on defender Florentin Pogba for midfielder Emerson Hyndman.

That switch doesn’t necessarily have to make Atlanta less dynamic, though. After moving to four defenders, Atlanta quickly scored the goal they needed in the 1-0 win over New England.

De Boer’s main conundrum is that there’s no obvious choice – to go with three defenders, he’d have to utilize Pogba or Leandro Gonzalez-Pirez in the middle out of their natural defensive positions, or perhaps even try to use midfielder Jeff Larentowicz there. None of those are ideal.

To go with four, De Boer needs to shift his attacking formation around, possibly moving Julian Gressel or leaving Emerson Hyndman out of the starting lineup, and have his team play a style that’s at least slightly different from what they’ve done all year.

It’s not an easy choice to make. Whether De Boer gets it right or not will likely dictate if Atlanta’s title defense continues or ends against a team they lost to 3-1 the last time they met.

For what it’s worth, in that match, in August, Atlanta used three defenders.

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