I can't determine from this video as to where the actual injury occurs. It looks like contact is made while Marc is standing, but the injury is to his right knee and it doesn't appear anyone gets close to that side. Likewise, when Gasol goes to the ground there isn't any further contact with the Rockets players.
Maybe it's possible that Marc's own legs hit together on the way down, or that there was some contact with the court that's to blame.
However it happened, we finally have an update from the MRI:
Marc Gasol has a right knee bone bruise. He will be listed as questionable for game Friday vs. San Antonio Spurs— Ronald Tillery (@CAGrizBeat) December 4, 2014