And there was no place for Moeen in the 12-man squad for the upcoming two-match series against Pakistan, which was released on Tuesday.
Wicketkeeper Buttler, who has played 18 Tests for England, the most recent in December 2016, is expected to bat at seven.
Buttler, who is now playing in the Indian Premier League with Rajasthan Royals, has not played a Test for England in 18 months.
But he has not played first-class cricket for Lancashire, his English county, since the end of last season.
The 20-year-old Bess is likely to make his global debut at the Home of Cricket after Leach's unfortunate injury.
Bess, who has taken just one wicket in two Championship matches for Somerset this season, said he thought he had a "slight chance" once news of Leach's injury broke but wondered whether the selectors might have opted for someone else, perhaps Amar Virdi who has been in wonderful form for Surrey in the Championship.
20-year-old Bess seemed to have received call-up on the back of his impressive performance in the Champion County match for the Marylebone Cricket Club (MCC) against Essex in March, when he bagged eight wickets besides scoring a century to help his side come out victorious.
"Dom Bess has made an excellent start to his first-class career", Smith added. He has impressed everyone with his performances for Somerset and when he has been given opportunities with England representative teams.
'We felt this was the right time to bring in a player of Jos's unique gifts, what he brings to the team as a destructive batsman, a fantastic competitive presence and his innate leadership skills, when he was playing with such confidence and flair, ' said Smith of a batsman excelling in the Indian Premier League.
Buttler has already played three Tests as a specialist batsman, since he lost the Test gloves to Jonny Bairstow.