It would seem reasonable that Isaac is back and starting in 2022 given that his injury occurred before the beginning of the 2021 season (or earlier??), and if Tarburton comes back he also likely starts. But as for the backups to fill a 4-5 DE rotation, it will be interesting to see how that shakes out.
Vilbert who will be a redshirt Jr in 2022 got playing time this season and seemed somewhat improved from his RS-Fr season of 2020. Fisher (RS-Fr this season) got some playing time this season but didn't seem to show up much in the stats, tackles, tfl's, ... I didn't notice the other RS-Fr Mostella getting into any games, and he is still listed as pretty thin (229 at 6'6"). Then there is the 2 true freshman that redshirted this season, McGraw and Townley.
So there are a good many DE's that could jump into the competition. But if the staff does not choose to bring in a transfer that may be because they also believe that DDS is up to the challenge of being the 4th or 5th DE for 2022.
If we don't see a Portal DE show up over the next month, that may signal that PSU is pretty confident that Isaac is on track with his recovery and is expected to be ready to go for Fall practice.....