There are a host of reasons to explain Mariano Rivera’s fall from nearly perfect to merely good.

Too little use, too much nibbling, with a tiny mechanical flaw thrown in for good measure.

Whatever the cause, the result is this: The Yankees closer hasn’t been his dominant best this season.

After allowing a run in two of his past three games, Rivera, who posted a career-best 1.38 ERA last year, saw this season’s ERA rise to 3.18. The seventime all-star hasn’t pitched to an ERA over 2.00 since 2002, and hasn’t been over 3.00 since his rookie season (1995), but he refused to use rust as an alibi.

“I don’t attribute that to anything. It is what it is.

I’ve been having a tough time locating the ball,” Rivera said. “I don’t want to blame it to anything, [say] because I don’t pitch, I’m not doing it. No – it’s because I haven’t been able to make my pitches; that’s it.

“I’ve been working on mechanics, on my release.

Even though I gave up that run, [Tuesday] was better.

The ball was moving and I was able to hit my spots. It was the fastball away; if I went away, I was leaving it right over the plate and up.

I was 0-2 on guys and I couldn’t finish them because I throw [to] the outside and it’s sitting on the plate. That’s definitely a problem you have to fix.” With foes batting .299 against Rivera coming into last night’s game vs. Texas, he had allowed 20 hits and six earned runs in 17 innings, too many scores and too few frames. After working three straight days, he was idle for three more before allowing a run Tuesday.

More alarming, his strikeouts per nine innings were down to 4.24, nearly half his career average of 8.04; and his hits per nine innings were up to 10.59, far more than his career mark of 7.10 or his nonrookie high of 8.16.

“Whether he’s missing outside, maybe you’re trying to be too good instead of just throwing it out there,” pitching coach Ron Guidry said. “When pitchers try to make too perfect a pitch, we don’t throw it as good as [we] should.

That’s the one that might get hit.

“It’s not like he’s getting pelted all over with line drives. Balls are falling in, dribbling, so he’s not far off.

“In all fairness, he pitches once every three or four days and we haven’t had a chance to put him in there regularly.

He hasn’t had a chance to be used like he should be; but hopefully that’ll come.” Joe Torre may have described the Yankees’ faith in Rivera by saying, “I think if we put him out on the market, we’d have people calling us.” “He hadn’t worked in a few days. It’s been a little feast or famine for him [in terms of getting work],” Torre said. “Some days are better than others. He has no issues at all.”