Webb damaged his ankle in the same game for Wales and Gatland has opted to call up Phillips, a veteran with two Lions tours and 94 Wales caps under his belt, as scrum-half cover.
The double loss is a tough blow for Wales - who are paired with England, Australia, Uruguay and Fiji in Pool A - but Gatland is not too downhearted.
And Wales head coach Warren Gatland said: "It is extremely disappointing for both Leigh and Rhys.
"After announcing the squad we spoke about the need for players to remain ready for action and it is a great opportunity for Mike and Eli.
"It's great that we can call on players of this calibre. To bring Mike's experience into the squad is a bonus along with Eli's exciting talent."
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