There have been seven instances over the past 27 years where the two teams that met in one of the conference finals faced each other on Opening Day the following season. Here is how they fared in those Week One contests, with the first team listed being the one that won the conference title:
1976 Steelers @ Raiders 28-31L
1978 Broncos vs. Raiders 14-6W
1980 Steelers vs. Oilers 31-17W
1983 Redskins +3 vs. Cowboys 30-31L
1991 Giants -1.5 vs. 49'ers 16-14W
1999 Falcons +4 vs. Vikings 14-17L
2002 Patriots +3 vs. Steelers 30-14W
As you can see, even the teams that lost did so by three points or less. And the three teams on that list that won as Super Bowl underdogs like the Bucs did, ('83 Skins, '91 Giants & '02 Patriots) all covered the number. Good luck whichever side you're on.
Big Lou