I don't think so, not this time. Although I could be wrong, I think Jimmy's ratings will be better than Jay's overall, and I think especially so in the younger demographics. If this happes there is no way NBC would want Jay back. Hell, even if Jimmy's rating are the equal of Jay's, I don't think they'll ever have the little weasle back.
Jimmy is also doing alot of pop culture things people actually understand
-He just did that Full House reunion skit. That just make older fans all nostalgic and laugh at their childhood.
-He did that history of rap with Timberlake that struck a chord with 25 to 30 year olds.
"OMG! I remember that beastie boys song!"
-He does alot of things young people understand
"You're mom accidentally wiped your PS3 save file etc because she thought it was dirty stuff etc etc.
Things like that
His first guest on The tonight show will be Will Smith. Expect a "Fresh Prince" moment and maybe a rap battle.
Ratings will be huge.