Hi Anjum,
When I said that the appointment of a devil can apply even to 'Muslims', I made that statement based on the premise that not all Muslims are following God the way He is supposed to be followed. It is not about merely being a believer or a non-believer, but it is also about following the right guidance in the right way. Many Muslims today believe in God and in His Quran, but are they free from associating partners with God? The answer is arguably no. Idolatry from a Quranic perspective is not restricted to just statue-worshiping, but it can take other forms as well. Brother Joseph has an article regarding this matter. The link of the article is provided below
[1]. Hence, it is these people who are believers only on the surface and are not pure believers, to whom surely devils can be appointed. They don't necessarily have to leave Islam altogether and / or abandon the salat.
But as for those who are following the guidance in the right way without associating partners with God in any way or in any form and who are true worshipers of God alone, satans won't have power over them:
38:82-83
(Iblis) said: "Then, by Thy power, I will put them all in the wrong,
except Thy Servants amongst them, sincere and purified (by Thy Grace)."
15:41-42
God said, This is the path which leads straight to Me.
Surely, you shall have no power over My true servants, except those misguided ones who choose to follow you.16:99-100
Surely, he has no power over those who believe and put their trust in their Lord;
he has power only over those who are willing to follow him and associate others with God.On the strength of the above verses, it can be argued that God's true worshipers can't be overpowered by satan as long as they hold on to God's true guidance. I don't think merely missing a prayer because of not being well would cause a devil to be appointed to you. You can still "remember" God when you are unwell, can't you? So according to my humble opinion, my answer is 'no' if you are a real believer who do not associate partners with God in any way or in any form.
Reference:[1] http://quransmessage.com/articles/idolatry%20according%20to%20the%20quran%20FM3.htm