Karthikeyan had been linked with a move to Force India, but despite praising the 35-year-old for being "exceptionally competent", Fernley admitted that the Indian driver would not be considered when the team decide on their lineup for next season.
Fernley told AFP: "Narain is an exceptionally competent driver. If we put him in our car this weekend he could do better.
"But with all due respect to Narain, can he compete with and outperform our drivers? It is not his fault because he's not had the opportunity that our drivers have had. Unfortunately, that is the way it is."
Karthikeyan will race in his home grand prix at New Delhi's Buddh International Circuit this weekend.