Norwich City, who host ninth-placed Everton in the league on Saturday, put in a late bid for the striker in the last transfer window that Everton turned down, but Martinez said on Thursday that he would reassess Naismith's situation in January.
"The offer from Norwich arrived very late and I wouldn't have had time to replace him," the manager told reporters at his pre-match news conference.
"I was honest with the player and told him he was too important to the club to be allowed to leave.
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"I didn't know at that stage how Arouna Kone was going to be, or how Gerard Deulofeu was going to settle in, but I had a good idea that Steven still had plenty to contribute, and so it has proved.
"He has been a consistent performer since I came to this club, but at 29 I can fully understand him wanting to go somewhere else if he thinks he can be the main player."
"I do not want to lose him but I understand his position.
"I think if Naisy was 22 years old he would be happy to fight for his place - you have that confidence at that age - but when you are 29 and not playing as often as you would like it is completely different.
"I will talk to Steven in January and see where we are. If he feels we can offer him something he will enjoy over the next six months then fine. If not, we will try to help him out."
Martinez confirmed that the trip to 16th placed Norwich had come too early for injured centre back Phil Jagielka (knee), but said the England international could return to the side after Boxing Day.
Midfielder James McCarthy is also doubtful and will have a fitness test to see if he has recovered from the hip injury he sustained against Bournemouth on Nov. 28.
(Reporting by Simon Jennings in Bengaluru; editing by Amlan Chakraborty)
