They were making good progress, coach Gatland said.
He anticipates replacing six to 10 players of his original 41-man squad. The Lions in 2013 in Australia lost nine. He's already lost two; England scrumhalf Ben Youngs was the first to withdraw for family reasons.
"We spend a lot of time going through the process of picking players, and trying to plan and prepare, so to lose someone is disappointing," Gatland said. "You have just got to move on."
Gatland said they were tempted to replace Vunipola with another No. 8, but decided they were well stocked with Wales' Taulupe Faletau and Ross Moriarty, and Ireland's CJ Stander. So they went for flexibility; Haskell could cover all three back row positions, plus he played Super Rugby in New Zealand for the Highlanders in 2011.
"I spoke to Billy, and he just felt that he couldn't give us 100 percent," Gatland said. "You have got to respect that decision he has made and him not wanting to let anyone down.
"I did try and persuade him to come over (to Ireland) and be assessed, but he was adamant he wasn't right. He is a big loss to us, but I fully understand the decision he made to not go on the tour. He's had this ongoing shoulder problem for five or six months. He made that decision for himself, and he was adamant it was the right call."
Gatland was crossing his fingers there were no more injuries among his Lions squad members playing this weekend in the Pro12 and English finals.
The Lions leave next Monday, and play the first match on their 10-match tour the following weekend against a Provincial Barbarians outfit.