A poor umpiring decision by Indian official S Ravi gave India the prized scalp of Kevin Pietersen, who was wrongly adjudged caught behind off Ishant Sharma's bowling.
"It is frustrating when decisions don't go your way but that happens in all forms of cricket, especially without DRS," he said of the decision as India took a 2-1 lead in the five-match series.
Cook also said that it was a good toss to win for India skipper Mahendra Singh Dhoni.
"It was a good toss to win. We found it hard to judge what a good score was, but all our top six got double figures, so we all got starts," he said.
Defending a paltry 156 was never an easy job and Cook said it would have been a "freak" result had they won.
"It would have been a freak result if we'd bowled India out for 155 on that wicket. The damage had already been done clearly.
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"We had to attack, but I can't criticise the bowlers or fielders. Clearly 155 was not enough. We didn't get enough runs," he said.
Cook said England's batting failure in the last two games (Ranchi and Kochi) was hurting the team's cause.
"It (Batting) is a concern of course. It is frustrating when you don't play to your potential as a batting unit and we need to do that if we want to win here," he said.
"We've got some quality players in our dressing room who can, on their day, win games for England and if we want to win we need to stand up as batsmen and deliver.