Saracens were crushed 30-10 by French side Racing 92 in the opening game of their European Champions Cup title defence on Sunday.
Defending champions Saracens were given a tough draw for the 2019-20 Champions Cup and will play Munster, Racing and the Ospreys in the pool stage.
England's Saracens were crowned champions of Europe for the third time in four years as they came from 10-0 down to beat last year's winners Leinster.
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".
Pat Lambie is almost certain to miss the Springboks' Tests against England after sustaining what is believed to be an anterior cruciate ligament injury in the European Rugby Champions Cup final.
Ireland head coach Joe Schmidt faced fresh injury concerns over Jonathan Sexton and Sean O'Brien who were both withdrawn in Leinster's defeat to Montpellier in the Champions Cup on Sunday.