It is highly unlikely that Tennessee would take Bigelow without talking to the orthopedic surgeon that did the knee repair to get not only his current condition but also a prognosis on how much of a recovery is likely and how long it might take to reach a full recovery -- if ever.
Of course that leaves two problems -- one is that the prognosis may not be accurate and the eventual outcome could be better or worse than the prognosis. The second problem is that even if the physical outcome is great, the psychological effects of two different injuries and surgeries on the same knee could be hard to overcome. He could be physically fine but not have the confidence to play full speed and without worrying about his knee.
So yes, the kid would be a big gamble, but some due diligence with the surgeon could go a long ways to reducing that risk and a potential redshirt year could give him more time to recover psychologically.
I suspect Dooley is covering all the bases.
mlsoft