Macauley Cook has retained the captaincy having led the side to third place in the recent Six Nations.
Webster said: "The team played so well during the Six Nations - they did everything that was asked of them - and it was always going to be hard to leave some players out.
"We had to look at versatility, especially in the backline, and that, along with some impressive recent performances helped Iolo Evans, Dale Ford and Lewis Robling to secure three of the final places."
The former Lions flanker was announced as Darren Edwards' replacement for the tournament in Italy earlier this week, though Edwards will remain involved in the coaching setup for the rest of the month.
"With five matches in sixteen days, it is important to have the right mix within the group," Webster said.
"The players showed their character throughout the Six Nations on and off the field, and I'm sure that will help them in the testing environment of an international competition.
"We know we will be tested physically against all three of our Pool opponents - Argentina, New Zealand and Italy.
"However, we have fronted up against both England and France this season and we have no qualms in that respect.
"We have two or three sessions a week together now until we leave on June 6 and with the Principality Premiership play-offs, some players won't get a break, but we are looking forward to getting re-acquainted with the boys and getting to know the new players."