Momsen, who plays Jenny, has been criticised for her off-screen behaviour and guest star Tim Gunn claimed last year that she was unprofessional on set.
Paolo has now told TV Line that the attacks are unfair.
"She's 17 years old," he said. "I don't think anyone has any right to say anything negative being that she - as mature and self-possessed as she is - is still young. It's not fair. No-one has a right to attack someone of that age."
Momsen has only appeared in a few episodes of Gossip Girl this season and Paolo admitted that he misses her "dearly".
However, he added that he expects Momsen to return in the future, saying: "Jenny's always lying in wait. She could pop out of the bushes with her machete of eyeliner at any time."
Gossip Girl airs on Mondays at 9/8c on The CW in the US and on Wednesdays at 8pm on ITV2 in the UK.