The Merseyside outfit announced on Monday afternoon that Koeman had left the club with immediate effect.
The Dutchman, who was hired by the Toffees just 16 months ago, brought in a handful of players in the summer transfer window, spending in excess of £100m, but the results failed to live up to expectation.
The 54-year-old was given his marching orders after guiding the team to just two wins in nine Premier League games, which has left them third from bottom in the table.
Everton have confirmed that Unsworth, who is the current Under-23s coach, will begin his reign on Wednesday night when the team face Chelsea in the EFL Cup.
The next league game for the Toffees is away to Leicester City on Sunday.