For much of this tour, it has been hard to work out whether England are waving or drowning. They have been open to trying new things, fought hard in four matches out of six, even managed to win a couple against an opponent adept at pulling England's coloured one-day trousers down. There have been several individual performances to savour, though heavy defeats in the second and sixth ODIs pointed to an overall gulf in class.

It is clear though that someone is in need of a lifejacket. When a series is lost, the captain is inevitably first in for questioning. Unfortunately for Alastair Cook, no one is interested in pursuing other lines of inquiry. Leaders can be totemic in defeat as well as victory and England's recent one-day record has become increasingly bound up with Cook's own desperate struggle for form.

Peter Moores, the head coach, has admitted that Cook is not guaranteed to be in charge for England's World Cup campaign, despite James Whitaker previously saying the selectors were "100% behind him". Moores continues to believe Cook is the right man for the job but said he would sit down with Whitaker, Mick Newell and Angus Fraser and "review the series at the end as we always do".

There is no question about Cook's thoughts on the matter. "Defeat doesn't finish a man, quit does. A man is not finished when he's defeated. He's finished when he quits." That was said by Richard Nixon but it seems to encapsulate the Cook position. His belief in his ability to turn things around is admirable, yet it is also beginning to make Cook seem crankish, even obtuse, a Nixonian figure railing against those who are out to get him.

"He is fine," Moores said. "Of course he is frustrated at not getting a score. But he is clear that he wants to carry on captaining England. He wants to take the team to the World Cup and he wants to win a World Cup...He knows nobody has got the guarantee of that."

The last few weeks have seen Cook teetering above the chasm. Had Joe Root and Jos Buttler not puckishly plucked victory out of the fire in Hambantota, England would have gone 3-0 down in the series. Cook could barely savour the moment before he was handed a suspension for England's slow over rate. He returned for the Pallekele win that kept alive faint hopes of causing an upset against Sri Lanka but there was no respite for Cook with the bat.

He has been able to treat triumph and disaster just the same, largely because his own returns have been so consistently poor. Sadly the imposter appears to be Cook as a one-day batsman. While England rearrange the deckchairs around their captain, an iceberg is looming.

The numbers make for grim reading. Cook's average on this tour is 17.40 and he is the only member of the current top seven not to have scored a half-century. In 2014, he averages 27.27, with a high score of 56 in 19 innings. It is 44 innings since his last one-day hundred, against West Indies in June 2012; in that period he averages 30.45 at a strike rate of 72.13. As a batsman, in this form of the game, his position is near untenable.

As a captain, Cook was never about rousing speeches or tactical masterstrokes but rather an ability to inspire through deed. His leaden example at the top of the order now seems to have the opposite effect. England have lost five ODI series in a row under Cook, offset only partially by a run to the final of the Champions Trophy. Their last bilateral win under him came in February 2013.

England have resisted calls to replace Cook from numerous former players, including Graeme Swann, Michael Vaughan and Ian Botham, partly in the hope that he can regain the form that saw him average 54.47 at 91.59 in his first year as captain. The issue is also clouded by the fact Eoin Morgan, the most likely replacement, is in a rut at least as bad as Cook's - though he did score 62 off 47 balls when leading the side in the fourth ODI.

"For me, I think Alastair's the right man to move us to the World Cup," Moores said. "But like most things, I'll go into a room with the other selectors and we'll talk about it. We'll talk about our team. A lot's happened over the last six ODIs, a lot of good has happened. Then we need to formulate that into a squad of 15 people who can go to Australia and be competitive for the World Cup. That's what we've been working to and that doesn't change.

"We're trying to balance out the right team. Of course Alastair needs some runs but we're trying to balance out an aggressive batting order, a side that can take wickets throughout and also play in an environment that's great for young players to express themselves. We've got a lot of young players coming in to this team who are doing well.

"So you have to take all of those things into account, try and pick who you think is the best leader, the best team to be successful in Australia, the tri-series to build ourselves up to the World Cup. I'll spend many hours thinking about it and we'll discuss it with selectors and that's my opinion at the moment."

After his success in rejuvenating the Test team during the summer, England believe Cook can oversee the transition to a new-look 50-over side built around players such as Moeen Ali, James Taylor, Chris Woakes and Chris Jordan. However, the focus on Cook has overshadowed the progress being made.

"There is a bit of frustration that someone like Joe Root batted really well yesterday, got a stellar hundred a couple of days before, has had an unbelievable year of being in the top 10 leading Test run-scorers and also has three one-day hundreds," Moores said. "I understand that there will always be debate about players who aren't in form. And the captain gets extra focus. That goes with the territory.

"And I don't want people to say we are better than we are or give us stuff we haven't earned. Players have deserved real credit for some of the things that have happened on this trip. Some of Moeen Ali's shot-making is beautiful to watch. As he settles on his tempo, he is looking a very exciting cricketer with bat and ball. I think Chris Jordan is starting to express himself on an international field, Chris Woakes is starting to show that he can take wickets at this level and have an influence. They are exciting things for us. James Taylor came in and took his chance. So they are good things. You balance them all out on the trip."

Changing the captain two months before the start of the World Cup would not suit a team as meticulous as England. They set out on this path four years ago, under Andy Flower. But if they are intent on developing the kind of positive game that has been glimpsed in Sri Lanka, and which has given Moores such encouragement, hard decisions will also have to be made. The final match of a series that has already been lost does not usually have much riding on it but it could be Cook's last chance to prove that defeats do not finish a man.