The uncapped 24-year-old got the nod ahead of Tim Ambrose, James Foster and Chris Read on the strength of his recent batting record.
He has scored 893 runs in domestic one-day cricket this season at an average of 49.
61.
Mustard will fly to Sri Lanka on Sunday to join his new team-mates ahead of the five-match one-day series, which begins on October 1.
Prior was ruled out of the series after breaking his right thumb in practice at the World Twenty20 tournament in South Africa.
David Graveney, the chair of the England selectors, said: "Phil has been in outstanding form for his County this season, has experience of opening the innings in domestic one-day cricket, and this is an excellent opportunity for him to test himself at a higher level.
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