The thrashing in England, following the loss in South Africa earlier this year, piles pressure on Shastri ahead of a tour to Australia in November and next year s World Cup -- as well as a massive clash with arch-rivals Pakistan next week.
Critics and former captains have rounded on the coach, particularly for his mid-series claim that his team is the best to come out of India in 15 years.
"Self-belief is a wonderful quality; but being caught in the bubble of your own publicity is self-defeating," said Sambit Bal, chief editor of cricket website Cricinfo, on Twitter.
"India had their moments but the scorecards say 4-1 and India weren t as good as they should have been," said noted commentator Harsha Bhogle. "This is two overseas disappointments in a row now."
Prior to the final Test at the Oval -- which India lost by 118 runs -- Shastri had raised eyebrows with lavish praise for his side.
"If you look at the last three years, we have won nine matches overseas and three series. I can t see any other Indian team in the last 15-20 years that has had the same run in such a short time," he said.
India have won two recent series in Sri Lanka and one in the West Indies.
But his comments rankled former star players.
Sourav Ganguly, who was part of the committee that appointed Shastri last year, dismissed the coach s claims as "immature", telling Indian television Shastri must improve the team.
Sunil Gavaskar pointed out that Indian teams of the 1980s had won series in England and the West Indies -- as well as a series victory in England as recently as 2007.
"The best travelling teams are made by performances on the ground and not by sitting in the dressing room and talking about it," added former Test opener Virender Sehwag.
Captain Virat Kohli leapt to his coach s defence following the final Test on Tuesday.
Asked about Shastri s pre-match comments, Kohli said: "We have to believe we are the best side, why not?"
In a tetchy exchange, Kohli turned the question on a journalist, who responded "I am not sure". Kohli hit back: "You re not sure. That is your opinion. Thank you."
Shastri now has to immediately lift the side for the six-nation Asia Cup starting in the United Arab Emirates on Saturday -- without the brilliant Kohli, who is being rested.
The whole country will be watching as they take on arch-rivals Pakistan on September 19, with Rohit Sharma to lead the side throughout the 50-over contest.
India then have two Tests at home to West Indies before four Tests in Australia. - APP/AFP