Football: Football players get injured all the time, often have several injuries throughout their career. They also come back from injuries and continue playing all the time. Adrian Peterson was injured every year in college, and got hurt even more as a pro. He had a pretty stellar career. And Adrian Peterson isnt the only example of coming back from injuries and playing well, ridler80 above shouted out to Frank Gore as well.
Add to it that Jackson is a fourth round pick. If you expect 4th rounders to come in and be huge contributors, then you know even less than I had originally thought. Anything that 3rd day picks give you outside of ST is a boon, that's why people get all surprised when late round picks end up doing anything. Sidney Jones was injured much more recently and got taken in the second because they expect a full recovery, so apparently taking players who got hurt before is a concept foreign only to you.
Medicine: Doctors are on record saying they expect a full recovery from his surgery to his broken leg. He came back from the ACL injury and had a good year with no lingering difficulties. Reinjury rates go down with advances in medicine. Also, the injury concerns that people fret over are the ACL and broken leg, I havent seen anything about back surgery, and that isnt to say he never had back surgery, but it doesnt appear to be concerning at all. So he got his medical attention.
Statistics: Show your math why he has a <5% chance of staying healthy. Take your time, I'll wait.
In sum, injuries are a part of football, nobody expects 4th rounders to amount to much to begin with, you failed math, go be a cunt somewhere else.