India captain MS Dhoni admitted his side may have been complacent ahead of their 41-run loss to England in Leeds on Friday.
Dhoni's men led the five-match series 3-0 heading into the final clash at Headingley but were below-par as England won their first one-day international since May.
Joe Root's 113 led the hosts and despite some late hitting from Ravindra Jadeja (87), India never looked likely to chase down 294 and were bowled out for 253.
When asked if India may have taken the game lightly with the series already won, Dhoni said: "To some extent, it may have been the case.
"That's one area where we have to improve. In the past when we have won the series, we have not done so well in the last game.
"I think we gave away too many wickets. I think 300 was a par score on that wicket. It was a good wicket.
"We were not really able to score the amount of runs we were supposed to. Shikhar [Dhawan], me, [Ambati] Rayudu, those are the wickets we should not have lost at that point in time.
"It's the soft dismissals that really let us down."
Dhoni has targeted India's late-order bowling as an area for improvement after England smashed 143 runs from the last 15 overs.
"The last 10 overs of our bowling is very important," he added.
"We can't really rely on spinners to bowl the last few overs. They [pacemen] need to step up a bit.
"I think the batting looks quite settled. Maybe 12 players have selected themselves for the World Cup [already]."
Suresh Raina could only add 18 runs and one wicket on Friday but was deservedly named the player of the series.
And he claimed a switch in the dressing-room atmosphere " following India's 3-1 Test series loss to England " had greatly contributed to their turnaround.
"I was watching the Test matches on TV. At the same time, the team was going in a bad phase," he said.
"[But] the World Cup is around the corner. You need to think positive. We got the belief in the dressing room, we got in a position to believe we could win from here.
"We had a great atmosphere in the dressing room. I think we have done really well in the fielding department."