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Had the situation been reversed I can almost guarantee the Chiefs take the ball first. Plus, if they won the toss in that game, they for sure would have gone first for the reason above.
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Andy Reid said they would have chosen to play defense had they won the OT coin toss.
Not sure if is was Reid or Mahomes (or someone else) that said had SF scored a TD on the first possession, KC would have gone for 2 if they scored a TD of their own. In fact, going for 2 in that scenario is pretty much a no brainer, since SF subsequently would have only needed a FG to win.
Shanahan’s decision is defensible, but definitely the riskier one.