Ashes: Alastair Cook's form worries Graeme Swann
England opener Alastair Cook needs a big score to convince him to carry on playing, says former team-mate Graeme Swann.
Cook, 32, has managed only 83 runs in six innings during the Ashes series.
"I've spoken to him and I'm torn because I know how determined he is," ex-spinner Swann told Test Match Special after England closed day four of the third Test on 132-4, still 127 short of making Australia bat again.
"But I don't know how hungry he is to carry on. I can't see a great deal of longevity in his career."
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The left-hander has 11,712 Test runs - more than any other England batsman - and is ninth on the all-time list.
Before the Perth Test, Cook said he had not made any decision on his future and he has a "game-by-game" approach.
At the Waca, the former captain became the first England player to reach 150 caps and the youngest from any Test side to reach the landmark.
The Essex batsman has made scores of seven and 14, the latter coming as the tourists - already 2-0 down in the five-match series - battle to avoid surrendering the Ashes at the earliest opportunity.
"It's not been a good enough return for a world-class player," added Swann, who played under Cook when England won the Ashes in 2013.
"He looks tired, but he does still want it. He's still desperately trying to do well - it's just that he's in horror nick."
Cook made a double hundred against West Indies in August, but has now gone 10 innings without making a half-century.
Swann added: "He's got a young family, he's got his farm. I know if he goes back to England and starts playing for Essex, he'll score four hundreds in the first five games of the season and think 'hang on, there's plenty left in the tank', because he is still young enough and fit enough.
"These first three Tests in Australia have been very bad for him, but I still can't see a viable alternative, which muddies the waters again."
Along with Cook's struggles, his successor as captain Joe Root is only averaging 29.33 with the bat in this series.
Australia captain Steve Smith has scored 167 more runs than Cook and Root combined in the series so far, but both were backed by England batsman James Vince.
"If they don't score runs in one innings, you expect them to do it in their next innings," said number three Vince, who made 55 on Sunday.
"They have shown in their careers they don't often have long periods without scoring runs.
"It won't happen for them all the time - but I'm sure they are both mentally strong and capable."
Former England opener Graham Gooch, a long-time coach of Cook, expects the 32-year-old to be part of the tour of New Zealand in March and April.
Gooch told BBC Radio 5 live's Sportsweek Cook is "not old for a Test player", adding: "He'll play out the rest of the series and I imagine he will play in New Zealand.
"If the bad run continues it would be up to the selectors, but you go through phases in your career. He can turn it round, there are still more runs in the tank if he has the desire."
Jonathan Agnew, BBC cricket correspondent
Alastair is a very stubborn man, one of the most stubborn I have dealt with in this sport.
He is very determined and he would really hate to be driven out of the team because of a lack of form. I'm sure he would jump before that happened.
He is a cricketer with a very grounded family life - he lives on a farm, has a wife and kids - and he's very settled and happy with that life, so he's not one of those who will cling to his career with his fingernails.
If he said at the end of the series that this was his lot, it wouldn't surprise me.