Man United faces most important transfer decision under Louis van Gaal

The last time Manchester United didn't have a proper succession plan in place, they went six long years without a reliable goalkeeper. So if David De Gea leaves this summer, as expected, United had better get the choice of his replacement right.

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De Gea is only 24. It seems like yesterday that United brought him in to take over for the retiring Edwin van der Sar in 2011. He has been the starter for four seasons since then. And after a shaky beginning, he was voted in the Premier League's Team of the Year in his second and fourth seasons, was named the club's Player of the Year in his third campaign and the players' Player of the Year in 2013 and 2014.

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Sir Alex Ferguson, who signed him, inarguably got it right. De Gea's signing was seen as risky, though. He was young, and he'd only been Atletico Madrid's full-time No. 1 for a season. But, as Ferguson told me in an interview back then, he'd made the error of deeming Petr Cech too young, and he'd gone on to excel at Chelsea. He wouldn't be repeating that mistake again. And he didn't.

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Now, United has somehow let De Gea's contract run down to just a year, meaning the player has all the leverage – he could threaten not to resign and leave for free next summer, thus forcing a transfer. Real Madrid appears to want him to succeed the embattled Iker Casillas. De Gea, it seems, would quite like to go back to Madrid, his hometown.

"He has to decide," United manager Louis van Gaal recently told reporters. "Because we have [offered] a fantastic contract because we want that he stays. Fantastic season, I think. But I analyze also his position. And then I say, he's Spanish, he's a Spanish international – still behind Casillas – now a Spanish club is coming [for him]. His girlfriend is Spanish, his father and mother come here every week, every two weeks for him. And still it is difficult for him."

"He shall not leave us so easily because we have a great club," Van Gaal continued. "But he can go to another great club and that is for him to decide."

That doesn't sound very hopeful.

And it'll make United fans break out in cold sweats. Recall, after all, what happened after Peter Schmeichel, probably United's greatest ever goalie, retired in 1999. Remember Mark Bosnich? Or Massimo Taibbi? Or Raimond van der Gouw? Or Fabien Barthez? Or Roy Carroll? Or Ricardo? Or – gulp – a young Tim Howard? Or Nick Culkin, Paul Rachubka and Andy Goram? United fans will have banished those names from their memories for the sake of protecting their own psychological well-being. But all of those men made starts for the Red Devils, as Ferguson searched feverishly for a decent and reliable goalie, before he finally signed Van der Sar from Fulham.

Also consider this: While United suffered through its revolving door between the pipes, it had one of the best back lines in the world, and a merciless enforcer patrolling the space in front of them in Roy Keane. They don't have that luxury now. In fact, in whatever combination you compose a back line from Van Gaal's grab bag of defenders, there's no disguising its weakness. United needs to buy a good defender or two but will have trouble doing so. Elite defenders, as ever, are scarce and therefore ever so pricey.

There seem to be plenty of solid options for De Gea's replacement. United signed Barca's Victor Valdes on a free transfer from Barcelona this season, and he's strong with the ball at his feet, which is important to Van Gaal. Valdes also has just a year left on his contract, though.

Other targets could be Tottenham Hotspur's Hugo Lloris or Bayer Leverkusen's Bernd Leno. A few other names have been mentioned – Asmir Begovic of Stoke City, Samir Handanovic of Inter Milan – but the easiest signing could prove to be Cech, who was been displaced by Thibaut Courtois at Chelsea this year and wants to move on.

If Van Gaal is forced to anoint a successor, it could be the most important transfer decision he makes in his time at the club.

Leander Schaerlaeckens is a soccer columnist for Yahoo Sports. Follow him on Twitter @LeanderAlphabet.