This would be a fine way to regenerate. Killed by a fog-minded friend. Not that he wanted to. Especially with how things with Rose were going... and the fact he had met the idiot who would replace him. Not to mention he wasn't even sure if he could regenerate in a paradoxical place like this. Oh, that was a scary thought.
All that rushed through the Doctor's mind in the time it took K9's shot to connect with and cause Jake to release his grip on the Time Lord. Which, of course, meant a sudden four foot drop to the ground. If he's mind was functioning properly his time senses probably would have allowed him to prepare and land better but fogged up and in pain as he was, the Doctor simply landed hard on the floor. His right leg taking the brunt of the fall.
Soft words of lilting Gallifreyan left him at the impact before he quickly shifted in case he had to move fast. Pain once more pushed aside like only a Time Lord could.
"Recalibration complete," he heard K9 announce and he looked over to see the laser beam shoot out from K9's nose straight at Jake.