NFL - down by contact

[jack015]

I saw this happen in a game last week.

Legal forward pass beyond the line. Receiver and defender contact each other while both are making a legitimate attempt to catch the ball, Neither commits PI. The pass is tipped into the air. The defender, as a result of the incidental contact falls to the ground and while sitting on the ground, intercepts the pass. He then gets to his feet and advances.

My question is: Does a player heve to be in posession of the ball when the contact occurs to be considered "down by contact"? Based upon the way this play transpired, it appears that this is the case.