I wouldn't be too worried. The last two seasons his ERA's were higher in April -- granted, not his current 7+, but higher nonetheless. Perhaps, as he gets older, he is starting slower. He blew a save in April last year and two in April 2005. He'll warm up with the weather. He hasn't been pitching too badly.
It is a small sampe size -- he has only made 7 appearances to date, but let's look at some defense independent indicators:
His K/9 is excellent at 9.95 and above his career average.
His BB/9 is only slightly elevated at 2.84 (his career BB/9 is 2.31)
Here's the scary part: his groundball rate fell off a cliff -- GO/AO 0.57. The last three seasons his GO/AO rates were 2.16, 1.44, and 2.33.
But, that should right itself, I would attribute that to simply a rough stretch and a poor sample size. I can't see his K rate increasing, his walk rate remaining normal and suddenly everyone hits the ball in the air on him. erhaps it indicated a correctable mechanical flaw that is making his pitches end up high in the zone. Also, since he has only given up one homerun, his GO/AO rate increasing doesn't seem to be anything but an anomaly and certainly not his problem. If you remove that one outing were Marco Scutaro guessed right and hit that one homer, Mo's ERA would be a respectable 3.18.
He has had two bad outings in seven appearances, unfortunately those two happened to be his only two save opportunities. But, certainly Rivera didn't suddenly develop an aversion to pressure. I could see him getting older and losing his effectiveness, I can't see him suddenly become a choker.