The last thing McDermott wanted to do was to give the ball to the Colts at the 42 with 4:00 left. No chance of a safety there and even if you get a three and out you more than likely won't get the ball back with better starting field position than the 20-25 yard line. I doubt McDermott thought the O was capable of going that far in two minutes.
Granted, the offense went nearly that far to win it thanks to Thompson's catch but I don't think McDermott thought the Colts were going to get all the way to the 31 yard line after the punt.
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Maybe he had too much faith in his defense.