"Massive as it has always been," Shastri told reporters on the eve of team's departure for England and Wales to play the World Cup starting on May 30, here on Tuesday.
"He is right there, his communication has been fantastic. As a keeper, he has shown over the years that no one is better than him in this format, not just in taking catches, inflicting those run-outs or stumping. These are little moments in the game that can change it on the head and there was no one better than him," he said.
Dhoni will be playing his fourth World Cup and looked in great touch in the Indian Premier League (IPL) 2019, amassing 416 runs at an average of 83.2 and leading Chennai Super Kings to the final.
"It is great to see the way he was moving in the IPL, especially when he was batting. The footwork, the power with which he was hitting the shots, he will be a big player in this World Cup," Shastri said.
Talking about the format of the World Cup, Shastri said it would be the most challenging keeping in mind the improvement of teams.
"It will be very challenging. If you look at 2014 and 2019 now, the gap has become closer between the teams. You see what Afghanistan and Bangladesh were in 2014 and what they are now. It's a strong competition. West Indies, on paper, is as strong as any other team," he said.
Shastri said India would have to be flexible since overcast conditions could change things quickly in the UK.
"I think our mantra will be to be flexible according to the conditions. Because this is one country in the world where the pitches may be flat but if there is overcast condition then you have to be up for it," he said.
"In no other country, overcast conditions matter so much. London might not make that difference, but if you go up north and it does get overcast then you will see things happen. We have to be ready for that and prepare to be flexible," the coach said.
"This team has played good cricket in the last five years. They have been striving for consistency. They will not play any differently because it's World Cup. They need to enjoy it and if they play to their potential the Cup can be ours," he said.
Talking about the bowling attack, the coach said, "This is an experienced unit. They have been together for the last 4-5 years. They have bowled as a unit together. They accomplished what they set out to do in Tests and the endeavour will be the same here as well, to complement each other and keep the pressure right through the 50 overs."
Shastri also batted for World Cup to have play offs like the IPL.
"I have always said that. That's why the IPL format is fantastic. You never know in future, down the line, they (ICC) might think of doing it because of what exactly you said. You know that can happen. But at this moment, we will have to just say bad luck."