The 31-year-old batsman was dropped from the England side during the recent end-of-summer series against India, sparking reports that his 50 overs career was coming to an end.
However, Pietersen was included in the 15-man squad for the return series with India later this month and Cook has made it known the quality that his South African-born teammate provides.
"I see Kevin as a huge part of this one-day side, especially for the 2015 World Cup," Cook told Sky Sports News as his side prepared to depart from Heathrow.
"He has had a tough 12-18 months with his form. This happens when you spend a huge amount of time at the top of the game, it's very hard to keep your standards high.
"I think he still averages high-40s in Test cricket, so it is amazing we are talking about dropping that when he is still a fantastic performer."
England play their first warm-up match of the tour this Saturday.