Zak Crawley's success may well have ended Joe Denly's Test career

I think Jos Buttler is safe, but he shouldn't be - Dobell (2:00)

George Dobell thinks that Jos Buttler will continue to be picked in the England Test side despite poor performances. (2:00)

Joe Denly could be forgiven for some mixed emotions as he watched England bat on Saturday afternoon. As a fine team player and long-term advocate of the talents of Zak Crawley he will, no doubt, have been delighted by much of what he saw.

But, on a personal level, he may have been kicking himself for missing out. For the fact is, Crawley's success may well have ended Denly's Test career. Barring illness or injury, it seems one of them will have to miss out in Manchester to make space for the returning Joe Root. And with Crawley making the highest score of the match to date, he would appear to have made a compelling case for his retention.

Denly came into the game in the stronger position. He had been given the No. 3 position, after all, while Crawley was asked to keep Root's No. 4 spot warm. But Denly failed to make 30 in either innings. And now, after 15 Tests, his average has dropped below 30 and he remains without a century. It's not a small sample size and it's not a good record.

Crawley, aged 22, is 12 years younger than Denly. As an opener by trade, he is probably better suited to the top-three role currently required by England and, underlining the impression he is learning every day, he has improved his Test best score in each of the five matches he has played.

He is not, by any means, the finished article. His dismissal here, the over after Ben Stokes, was disappointingly soft: a clip across the line that produced a slice back to the bowler. In such situations, he has to put a higher price on his wicket. His departure was the second wicket in a spell of five for 30 that brought West Indies roaring back into the match. But whereas Denly has sometimes looked hurried by quick bowling, Crawley seems to have more time, more strokes and more potential. Put simply, he has a higher ceiling to his game.

"As soon as Ben got out it was clear I had to try and bat through to the end," he said afterwards. "To get out the next over was extremely disappointing. Hopefully if I get another chance I can do better.

"If we can get above a 200 lead it will give us a great chance to win the game. The pitch was offering a bit with up-and-down bounce. We've definitely got the bowlers to take 10 wickets."

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For better or worse, the Ashes remains the barometer against which England's Test team is judged. So it is relevant that Crawley looked especially comfortable in Johannesburg, on one of the quickest wickets in the world, and in conditions that perhaps most resemble what England can expect in Australia. In the first innings there he made 66, with all 11 of his boundaries coming in front of square. He pulls and hooks especially well.

Part of the attraction of Denly is his record of seeing off the new ball and wearing down the bowlers. While he has failed to convert his starts into substantial personal scores, he has played an important role at times by making life easier for those batsmen who have followed him. At one stage, he faced a minimum of 100 balls in nine innings out of 13. He has, at times, filled the Trott-shaped hole that remains in the England side during a tricky era of top-order batting. He deserves credit for the bravery and dedication he showed in achieving that.

The evidence that this has led to strong positions for England is debatable, though. His top scores over the winter - 74 in Mount Maunganui and 50 in Centurion - both came in defeats, while his scores in England victories were 38 and 31 (in Cape Town), 25 (in Port Elizabeth) and 27 and 8 (in Johannesburg). He played his part, no doubt, but if those scores are the bar by which England are judging their top-order players, they have a problem.

Denly's dismissals here have not been encouraging. In the first innings he was bowled - not for the first time - by one that found its way through the gap between his bat and pad. It is a long-standing technical fault - it was clear when he first appeared for England in 2009 and soon after in 2010 - and it is clearly being targeted by opposition bowlers. In the second, he clipped an innocuous delivery to mid-wicket. A soft dismissal, certainly, though he was not alone there.

The relative solidity of England's opening pair perhaps works against Denly, too. Although he didn't convert his good starts in either innings, Rory Burns looked admirably solid in both. His career average remains modest - it's 33.90 after 16 Tests - but, since the start of the Ashes nine Tests ago, it is 44.06. He looks as if he is finding his feet at this level.

Dom Sibley is not quite at the same level. He scored only three of his 50 runs in front of square on the off side and he has been dismissed by catches down the leg side too often to dismiss it as a coincidence. But his patience, his concentration and his recent record - he averages 40.11 in his four Tests this year - suggest he will win a prolonged opportunity to show what he can do.

All of which means the requirement for a batsman like Denly, whose main virtue is occupying the crease, may be diminished. Root, understandably, is keen to protect his free-scoring middle-order and has been appreciative of Denly's efforts to do that. But with Sibley and Burns finding their feet and Crawley adapting to this level by the game, he might feel now is the time to move on.

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There is a major caveat to all this. And that caveat is the Ed Smith factor. It is an irony that, when Smith was appointed as national selector, there was talk that he would take data-driven selection to a new level.

Instead, he appears to have favourites. His faith in Jos Buttler and Denly - who he played with at Kent and was touting as a spin-bowling allrounder in England's World Cup squad not so long ago - borders on zealotry. Between them they have now played 57 Tests and contributed one century. That's the same number as Ben Foakes managed in his first match. No amount of data twisting and sophistry will change that.

England cannot afford this fragility in their batting. While they fought back pretty well on Saturday and could even pull off a memorable win on Sunday, West Indies remain favourites. England cannot afford to go on investing in a 34-year-old who has obvious limitations and a relatively low ceiling. Crawley should be batting at No. 3 in Manchester.