Gloucester and England fly-half Danny Cipriani has been banned for three weeks after he was sent off for a dangerous tackle.
Deysel, who joined Ulster from Guinness Pro14 rivals Munster in 2017, made his decision because of a "combination of lower-body injuries and various niggles".
Danny Cipriani's week went from bad to worse as he was sent off against Munster in the European Champions Cup on Saturday.
England will be without Billy Vunipola for the autumn internationals after he fractured his arm in Saracens' win over Glasgow Warriors at the weekend.
Billy and Mako Vunipola both went off injured in Saracens' victory over Glasgow on Sunday, putting their participation in England's Autumn internationals in doubt.
Brian O'Driscoll has tipped Exeter Chiefs as favourites to win the Champions Cup title this season, alongside recent winners Leinster and Saracens.