Rather there is a good reason “to ensure within broad limits a certain uniformity of posture within the congregation for the various parts of the celebration of Holy Mass, and on the other, to not regulate posture rigidly in such a way that those who wish to kneel or sit would no longer be free.”
The answer, simply put, is “yes,” but it is also not merely up to the priest to decide. If by your question you mean that the priest sits before the eucharist is reserved by the deacon to the tabernacle than yes, that is OK. There is no prescription that states that he cannot sit and the faithful also do not have to take their lead from him. They can remain kneeling even while he is sitting.
If you are saying that the priest simply left the Eucharist on the altar and didn’t return it to the tabernacle before mass finished, than that is actually against the rubrics.
However, it doesn’t sound like that is your question:
Cardinal Arinze who is the prefect for worship at the Holy See stated that people do not have to all stand, sit, or kneel in unison after communion.
So if we have a bad knee we can sit. If we want to pray a bit longer we can continue to kneel. If everyone is standing it would be proper for you to stand also but if your disposition is more comfortable with kneeling or sitting than that’s OK too.
Mike Hayes is the Senior Editor for Googling God