The Yanks-Red Sox series ended Wednesday, so this is kinda stale; but I don't want to be one of those guys that only writes about his team when they win.
So the big story is that the Sandman hasn't done his game closing duty through the first series of the 2005 season, which makes people remember that he didn't quite manage to close things out his last two times out in Fenway last fall. People don't have to be terribly smart to see a pattern there (even if, as Brother Joe points out, that pattern's pretty darn flawed) which has lead people to gin up some theories.
A big, popular one amongst the New Englandy-types, is that the Red Sox own Rivera. I tend to doubt it, but it's not completely far-fetched. While player-on-player matchups (and team-on-player matchups) are often sample size illusions, every once in a while there's something to them: I remember the palpable fear a John Wetteland/Ken Griffey Jr. matchup would give me, back in the day. You just knew something bad was going to happen.
Sure, maybe Junior just got lucky. A whole bunch of times. Getting back to Rivera, it's possible that the Red Sox might've picked up on something the rest of the league missed--some pitch-tipping, or a pattern in the way Mariano throws. However, given the amount of yapping these guys did over the winter, if that were so, I think we'd've read it in a Gammons column by now.
Another, more likely, explanation is injury. He missed time with elbow bursitis this Spring, and was shut down over the winter, off his normal throwing regimen. Counting the postseason, Rivera threw 83 games and over 92 innings last year, the most he's thrown since 2001. For those of you that forget, after being the losing pitcher in Game 7 of the 2001 Series, Mariano had an injury-shortened season in 2002.
If we're talking about a bad arm, as a fan my biggest reaction is "better now than October". Given the number, if not the quality, of righthanded relievers on staff, the team could survive a short DL stay from Rivera, in the interest of having him healthy come crunchtime.
A more troubling thought is if you look at the pattern as: Rivera hasn't been the same since his relatives were electrocuted in his pool in Costa Rica. It's easy to give short shrift to players worries and emotional problems, because their effect on the field can be so hard to predict. Don Larsen, after all, pitched a perfect game the same day he was served divorce papers. Still, what can you do if the problem is that Rivera's head isn't in the game? There's no surgery for that type of wound, no cortisone shot that will grant temporary relief.
Still, the likeliest answer is that the season is still young. Rivera might be slightly out of condition, since he changed his winter routine, and lost time during Spring Training. In that case he'll get better as the season gets going.
After all, this is only week one.