He went the first 3 weeks with just 2XBH, that is certainly a slow start no matter how you slice it. His SLG was just around .300, quite awful out of a 3-hole hitter. He wasn't able to drive the ball with the injured thumb.
Papelbon is still a top 5. He's just not crazy dominating like Kimbrel, who has historic K/9 for a reliever. I'd take Kimbrel anyday and twice on Sunday over Papelbon.
Rivera does his thing with 1 pitch his career and still running strong. Papelbon might dance his act maybe 2-3 more year and tails off really fast.