No, it is not a catch because he didn't make a football move. The receiver had control - the back is the same thing as two feet.
See what I mean, it's not as simple as it seems.
You said "immediately". If the ball is "caught" and he's immediately hit and it pops out...it's not a catch. I see it every single weekend and just about every single game.
Actually, the back is the same as one knee in that situation (the two-feet part of the equation only applies if he hadn't been touched on the way down). If the receiver maintained possession of the ball through the contact with the ground, he would be down by contact and thus unable to make a subsequent "football move".