"Any player who gets injured at this time is a big loss to the squad," Gatland told British media.
"You've spent a lot of time going through the process of picking players and planning and preparing so to lose someone is disappointing.
"Given the history of the Lions we plan to lose anywhere between six and 10 players. That's just the attrition of past tours."
Vunipola is one of the most damaging ball runners in world rugby and Gatland said he had tried to convince the 24-year-old to travel to New Zealand despite the injury.
"I did try to persuade him to come over and be assessed and look at managing him, but he was adamant that he was not right," he added.
"If someone is not 100 percent right in terms of their commitment, I fully understand and respect that decision. It was pretty tough for him.
"He is a big loss to us, but I fully understand the decision he made to not go on the tour."
Gatland has already lost England scrumhalf Ben Youngs, who pulled out for family reasons and was replaced by Greg Laidlaw, and has a private list of players on standby in case of further injuries.
Haskell was devastated not to be selected in the initial squad after also missing out on the two previous tours but was not celebrating just yet.
"I am over the moon but so much can happen," Haskell said. "Until I'm on the plane, I won't really start that enjoyment..."
There was some good news on the injury front for Gatland, however, with Lions captain Sam Warburton confirming on Monday he had recovered from a knee injury that has kept him on the sidelines since April.
"Today I trained fully," the flanker said. "That's all the boxes ticked, and now I can crack on."
Haskell will not be in the first party of Lions players to land in New Zealand this week as his club Wasps have reached Saturday's English Premiership final against Exeter.
The Lions begin the 10-match tour against the Provincial Barbarians in Whangarei on June 3 with the first of the three tests against the All Blacks following on June 24.
(Reporting by Aditi Prakash in Bengaluru; Editing by Nick Mulvenney)
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