London - Kevin Pietersen believes it was a "poor decision" by England to drop Jason Roy during the Champions Trophy, a tournament he believes the hosts "should have won" in their own conditions.
Before the one-day competition got underway, struggling opening batsman Roy was given unequivocal backing by England captain Eoin Morgan, who said he would "definitely" stay in the side for the whole tournament.
But after managing just 51 runs in eight ODI innings so far this season, Roy was dropped for England's semi-final against Pakistan and replaced at the top of the order by Jonny Bairstow.
Test wicket-keeper Bairstow made 43 but England, previously unbeaten in the eight-team tournament, collapsed to 211 all out in a match eventual champions Pakistan won by a commanding eight wickets in Cardiff.
"Everybody struggles, it's a learning curve," said former England captain Pietersen, who has played alongside Roy at county side Surrey, at the launch of the South African Global League Twenty20 tournament in London on Monday.
Pietersen, one of the outstanding batsmen of his generation, added: "Everybody struggles, it's a learning curve. He's a youngster, he's not going to nail his whole career. You go through ups and downs, he's a kid.
"What he'll learn to deal with is how quickly to get himself out of this bit of bad form. Is he a bad player? No. He's a wonderful player. He shouldn't have been dropped. That was a poor decision.
"I think they should just have given him a run for the rest of the tournament. I'm not saying they would have won that game but they should have just stuck with it.
"They were scoring 300 regularly and then you drop a player and you get what you get. It's not probably because of him getting dropped, because I think Jonny (Bairstow) batted ok.
"I wouldn't have done it, but I'm not the coach or the captain."
The 26-year-old Roy, like Pietersen born in South Africa, returned to form with 92 from 81 balls in Surrey's One-Day Cup semi-final victory over Worcestershire last week.
Roy could return to England duty on Wednesday when they face the Proteas in the opening match of a Twenty20 series at Southampton and Morgan, England's white-ball skipper, said Tuesday: "He is still the same fella."
"He has been working tremendously hard. In international cricket, if you are striking the ball well or are in good form it doesn't mean you will score runs.
"He came back for his county and scored runs and the proof is in the pudding. He looked good in the nets and did all the right things but couldn't seem to score runs.
"It is impressive that he has come back from this run of bad form and hopefully he continues it throughout the summer, everyone goes through a bad stage but it is how they come out of it that is important."
England's only major limited overs tournament triumph of any kind came when Pietersen was a key member of the side that won the 2010 World Twenty20 in the Caribbean.
Hopes were high, however, that a team featuring Morgan, all-rounder Ben Stokes and fast bowler Mark Wood could lift the Champions Trophy and so end a 42-year wait for a one-day international success.
"England are a wonderful young team," said Pietersen.
"I know that India and Pakistan won't like it, but in strength in depth in English conditions, England should have won it.
"With the fire-power they have got and the players they have got, they should have won this tournament."