Because he fell awkwardly after his balance went from the punch Vitali landed on the top of his head, and his weight probably compounded the damage his knee received when he did so.
Notice how he is struggling to stand on his "good" leg (left) as he is getting up. Seems like he´s still wobbly. I think Solis is a great fighter though, exceptional talent. Had some nice counters in this match.
hes listed at 6'1". at that height he should weigh way less than he does now. explains y hes injury prone