Alternating goalkeepers, as has happened at three Premier League clubs this season, can upset defensive balance.
Plenty of twists for Premier League's men between the sticks
Goalkeepers are a bit like referees in that when things are going well you do not really notice them.
As a fan you want your goalkeeper just to be there, forgettable, his involvement barely discernible.
A fan may have a vague notion he is doing something useful, in terms of organising his defence; the former England goalkeeper Peter Shilton once commented that he had barely touched the ball in some of his best games because he had plugged gaps so well.
You do not want to see him. And you certainly do not want to be go to games wondering who you are going to see in goal that day.
Solidity and consistency are the watch words, yet this season three Premier League sides have vacillated alarmingly on their goalkeepers. Tottenham Hotspur started the season with Brad Friedel in goal. He played six matches, then Hugo Lloris came in for the win over Aston Villa.
After one match, Friedel was back and played four games before Lloris was selected for the defeat at Arsenal.
The France international has played in the four games since and would appear to be the first choice; certainly he is expected to start in tomorrow's home match against Swansea.
Rob Green has been restored to the starting line-up at Queens Park Rangers, who face Fulham today still looking for their first win of the season.
The England international played in the first three games of the season, making a dreadful error in the opener against Swansea, and was replaced by Julio Cesar.
When the Brazilian suffered an injury in Harry Redknapp's first game in charge, away at Sunderland, Green came in and has kept his place ever since.
To an extent those situations are explicable. Brad Friedel is 41 and it is understandable Tottenham should be looking to replace him.
His run of excellent form at the start of the season, though, made him difficult to drop and so the process of transition has been more protracted than was perhaps expected.
QPR's summer spending was scatter-gun and they have ended up with two very good keepers, albeit ones who probably hit their peak three or four years ago. One started the season poorly and was replaced; the other got injured and gave the first another chance.
What is going on at Manchester United, where Sir Alex Ferguson seemed unable to choose between David De Gea and Anders Lindegaard, is harder to understand. De Gea will presumably play against Sunderland today but there are no guarantees.
The Spaniard played in the first two games of the season but was then left out, seemingly because of a mistake against Fulham.
Lindegaard played in the next two games, and they then alternated for the next four matches. De Gea played the next five, Lindegaard one and De Gea three more before he had a wisdom tooth removed.
Lindegaard came back for five games before De Gea was recalled for the Champions League game against CFR Cluj last week.
He retained his place for the Manchester derby but may not have done had Lindegaard's wife not given birth. Having two players for any other position would be seen as a positive.
They could cover for each other, preventing burnout, offering an increased range of options.
But it does not really work like that for goalkeepers.
For one thing, they do not suffer the same wear and tear as outfielders running 12km or so per game. For another, one of the keys to goalkeeping is the relationship with the back line.
In the late 1970s and early 1980s, England had two excellent goalkeepers in Liverpool's Ray Clemence and Nottingham Forest's Shilton. Ron Greenwood, the manager, could not choose between them, so he alternated. The result was confusion.
"Liverpool pushed back onto Ray Clemence when opposing teams were putting them under pressure and gave him comparatively little space to come out for crosses," Gordon Banks noted at the time.
"But Peter Shilton always wanted that area immediately in front of him free. He's great on crosses and if the opposition have the ball in a wide position, and he can see they're going to hit it into the middle, he'll immediately push the defenders out to give himself room to take it."
When the ball was in the box, Clemence was reactive and Shilton proactive; when it was outside the box, Clemence effectively acted as a sweeper, at times, as Liverpool pushed out.
Shilton was less inclined to do that, preferring to hang back to try to psych out his opponent and draw an error from him.
Cameroon had a similar issue in the 1980s with Thomas Nkono, who liked to stay on his line, and Joseph-Antoine Bell who liked to leave his box.
With Nkono, the defensive line needed to be set deeper. As Bell pointed out, the two penalties England scored against Cameroon in the World Cup quarter-final in 1990 both came from fouls on Gary Lineker after he had run on to through balls in the space where he would have been sweeping had it been he, rather than Nkono, in goal. The line was too high for Nkono's style of play.
United have conceded 23 goals in 16 league games this season; they conceded only 29 in the whole of last season.
Ferguson has likened their defending this season to a"cartoon cavalcade" and vowed to improve things.
Consistency of goalkeeping selection might be a start.
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