Phillips, 25, is a former Wales Under-20 international who had trained with Wayne Pivac's senior squad.
The incident occurred on 5 December in Swansea, and Phillips said doctors were unable to save his leg.
South Wales Police said at the time that two motorbikes collided.
Phillips posted a picture to Instagram of himself smiling in a wheelchair and said support in the past fortnight had been "truly overwhelming" and his "spirits are high".
Phillips offered "heartfelt thanks" to those who helped at the scene of the collision and those at Morriston Hospital in Swansea who helped him afterwards.
"On the 5th of December I was rushed in to Morriston Hospital following a motorbike accident," he wrote. "I went into theatre on arrival but following ongoing surgery it was not possible to save my leg. As a result I had to have an above the knee amputation."
Phillips said donations to a fund set up to support his recovery would help both with his rehabilitation and "hopefully getting a prosthetic leg which will enable me to experience new opportunities".