Sarto out as Riccioni makes Italy cut for Six Nations
The 26-year-old Sarto has returned to his club Glasgow Warriors for tests on a dislocated right shoulder which may require surgery.
Scrumhalf Tito Tebaldi and lock Marco Fuser were the other players dropped from the original 34-man squad ahead of the opening games against England and Ireland.
Fuser, 26, has not sufficiently recovered from concussion suffered during a Champions Cup clash against Toulon.
However, Riccioni, a 20-year-old Treviso player, has been retained after joining the initial group as one of two invited players.
Riccioni, playing his first Pro14 season, impressed during the team's three-day training camp in Rome this week.
"It was a really positive gathering," said Irishman O'Shea, who is being assisted as Italy coach by former England manager Mike Catt.
"We wish Leonardo a speedy recovery from his injury and we have no doubt that we will need the entire group including players who have not been called up for these first two games, for what is an incredibly challenging tournament."
Italy – who took the wooden spoon for a second straight time last year – open against defending champions England on February 4 in the Stadio Olimpico and then go to Ireland for the second round on February 10
Veterans include captain Sergio Parisse, with 129 caps, and Leonardo Ghiraldini, 89 caps, with Alessandro Zanni set to return after a two-year absence as he targets his 100th cap.
The 33-year-old Zanni last played for Italy in the 2016 tournament but injuries have since prevented the Benetton flank from adding to his 99 caps.
O'Shea took over in March 2016 and his side shocked former world champions South Africa in November that year, beating the Springboks 20-18 in Florence. But the former Ireland international has had mixed results with Italy, winning just four of 17 matches.
Forwards: Simone Ferrari, Andrea Lovotti, Tiziano Pasquali, Nicola Quaglio, Marco Riccioni, Luca Bigi, Leonardo Ghiraldini, Oliviero Fabiani, George Biagi, Dean Budd, Federico Ruzza, Alexander Zanni, Renato Giammarioli, Giovanni Licata, Maxime Mbandà, Sebastian Negri, Sergio Parisse (captain), Abraham Steyn.
Backs: Edoardo Gori, Marcello Violi, Tomasso Allan, Carlo Canna, Ian McKinley, Mattia Bellini, Tomasso Benvenuti, Giulio Bisegni, Tomasso Boni, Tomasso Castello, Jayden Hayward, Matteo Minozzi, Edoardo Padovani.