23-year-old centre Cave had been ever present in the Ulster line-up this season, but missed last weekend's defeat to Biarritz Olympique with a hamstring injury.
After progressing through the Ulster Academy system, the strong-tackling midfield star made his Ireland debut in 2009 on the summer tour to North America.
Diack has had a big start to the season for the province, featuring in all but one of Ulster's matches so far and winning the man-of-the-match award for his performance in his side's opening Heineken Cup match against Aironi.
The South African becomes eligible to play for Ireland next summer having been a resident in the country for the last three years.
Moseley born Fitzpatrick impressed for Ulster in pre-season before suffering a calf injury but will push South African BJ Botha for the tighthead shirt on his return.