Ronaldo, 36, was named in the matchday squad but left out of the starting XI for the first time since he returned to Old Trafford as Everton came calling last Saturday.
Santos stressed Ronaldo's "rhythm" is of the utmost importance as he explained why he is likely to feature against Qatar despite the fixture only being a friendly.
"He will go to the game because he needs playing time. It's important to him at this stage," said Santos of Ronaldo on Friday.
"The last full game he played was in the Champions League. If he only plays against Luxembourg, he will have practically 15 days without playing with intensity.
"At this point I will say that there are strong possibilities of Ronaldo going to the game against Qatar, either starting or coming on later during the second half, so that the competitive rhythms are at their maximum against Luxembourg."
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