You are absolutely correct. Both Lars and Navarro are/were members of our 40 man roster. I imagine trading them was at least in part influenced by the 40-man roster crunch we were rumored to have. It was first reported a month or two ago but this is the only thing I can find mentioning it.
http://www.overthemonster.com/2011/7/28/2299814/mlb-trade-deadline-red-sox-rumors
EDIT: BTW, other prospects on our 40-man include:
Bowden
Doubront
Pimentel
Tazawa
Weiland
Exposito
Iglesias
Tejeda
Kalish
Reddick
We may attempt to trade one or two more of those players to free up roster space.