David de Gea: Manchester United's most talented player, and perhaps their only world class player on current form. An excellent team needs world class players across the pitch, perhaps three or four supported by others who are making the best of what they have. Right now, United have no other players of his stature or quality, and if they are to consider losing him, they should do so only in return for gaining a player of similar quality in goal or somewhere else on the pitch. Basically, whether they can get Gareth Bale or not. If yes, De Gea is replaceable in the sense that there are other 'keepers out there who wouldn't be out of place in a United side who could achieve things: Hugo Lloris and Victor Valdes, for example, would both be adequate.
That does not change the stone cold fact that De Gea is absolutely mustard, and with Edward Woodward's transfer track record, it is an enormous risk to sell such a talented player and expect his loss to be compensated for elsewhere on the pitch. If United have to keep Radamel Falcao for another season in order to prolong his stay for just one more year, then things become a little more complicated. Falcao is hugely unlikely to return to form and be the striker United need, but it can't be overstated how useful De Gea might be in what will be another season of accommodating another wave of new players. With defensive changes at centre-back and right-back certain, his ability to save points from defence would be almost essential. DON'T SELL. FOR THE LOVE OF GOD, DON'T SELL.
Anders Lindegaard: He was just enough when United needed a back up to play a few games a season when De Gea had the inevitable wobbles of a Spanish 'keeper coming to United at such a young age, but little else. Sergio Romero of Sampdoria was linked in the summer transfer window's closing stages, so it seems Louis van Gaal isn't keen. At 31, he isn't going to get markedly better, and has played just three games in the last two seasons. He's as handsome as they come, which is an underrated quality to have in a football side, but unfortunately God has not given with two hands where he is considered. BIN HIM.
Sam Johnstone: He's rated highly enough at the club to give him a chance, but in that sense he could benefit from experience on loan as a first choice 'keeper in the Premier League. Currently at Preston North End, he's played 15 league games in 2015. He's failed to score a single goal though, which indicates that Wikipedia's default statistics for goalkeepers might want to be changed to something more useful, like clangers or clean sheets. NO IDEA.
Victor Valdes: If he's willing to stay to play number two to De Gea for another season before taking over, then there's no reason to move him on. If he's required to step up as first choice at the start of the season, and his knee has properly recovered from being brutalised, then there are few other compelling choices elsewhere. Lloris is not especially convincing when playing high up the pitch, with established clownish tendencies, and Valdes has the experience from his days at Barcelona to make him suited to the style. DON'T SELL, ALSO ONE OF THE HANDSOME BRIGADE.