Sarri recently revealed that Moses would play as a winger this season after two years playing as a wingback under Antonio Conte.
Moses has found himself out of favour under the new Chelsea boss, playing just 11 minutes this campaign.
Sarri has switched to a back four since taking over at Stamford Bridge this summer.
The Italian explained Moses needed to get up to speed about returning to playing as a winger.
Sarri was asked why Moses was not playing and he replied: “Because he changed the position in the pitch. Now we are trying to let him to play like a winger.
“So we changed the position and in the new position I think he needs to improve.”
Moses was a winger before Conte deployed him in the wingback position and he helped Chelsea win the Premier League playing in that position.
Fellow wingback, Marcos Alonso has kept his place in the Chelsea side, despite Sarri changing the formation.
But it seems Alonso’s place is not safe as Sarri issued a warning to the Spaniard.
“He is a very good player and can improve in the defensive phase,” Sarri said about Alonso.
“Of course (I have told him). He has physical qualities at the top level.
I think that he can do more in the defensive phase. He is a good player in both but I think he can do more.”