At a meeting Wednesday of the Television Critics Association, Ferguson suggested that reviewers wait to sharpen their pens until Fallon adjusts to the new job. Fallon is expected to take over "Late Night" in March.
"I challenge you all to this: Give Jimmy a month before you review him," Ferguson told the group.
He questioned whether any critics thought he would last as host of CBS' "Late Late Show," which he took over four years ago. And he suggested Fallon, a former "Saturday Night Live" cast member, may be finding the late-night spotlight less comfortable.
There's been a bit of unfair negative buzz about Fallon before he's even started, Ferguson said.
Fallon is "kind of like the reverse Barack Obama. It's like he hasn't done anything yet, but everybody is commenting on his performance," Ferguson said. "Give him a chance."
Video blogs providing a behind-the-scenes peek at Fallon's preparations to take over for Conan O'Brien have been online since December.
Ferguson may feel he's in a position to be magnanimous, given that his show has been steadily gaining in the ratings on "Late Night."
On the personal side Wednesday, a reporter congratulated Ferguson on his recent marriage, to art dealer Megan Wallace Cunningham, and asked if she had any interest in being on his show.
"I can categorically say that I would not have married a woman that had any interest in being on my show," Ferguson replied, drawing laughs.