Northern Ireland defender Evans, 21, was regularly involved last season after spending the previous campaign on loan at Sunderland.
The centre back covered in Ferdinand's regular injury-enforced absences and made a total of 32 appearances as United retained the Premier League and reached the Champions League final.
He played a key role in the goalless draw at Inter Milan in the last 16, as well as playing against Porto and Arsenal in the quarter-finals and semi-finals respectively.
Ferdinand has again struggled with injury problems at the start of the current campaign and Evans has been a reliable deputy.
Ferguson admits Evans' fine form means that one day he will start key matches -- even when both Ferdinand and Vidic are available.
He said: "It is very difficult for me at the moment and I said to him last week that I don't know if I am doing the right thing by him by leaving him out because his performance level has been so high that at times I have been unfair to him.
"The one thing that justifies me leaving him out at times is the performance levels of the other centre-backs and their experience. Jonny is still young and has a great future ahead of him.
"The time time will come. The one thing about football is that young players in particular they come back from one summer and they are a different player with a different physique and they tell you by their performance that it is time to go in.
"It has happened at this club on numerous occasions and I would always think that that is the great worry for an experienced player that one of their own from their own academy or reserves is going to take their place."
Evans, who has now established himself as a regular in Nigel Worthington's Northern Ireland team, admits that last term was a breakthrough campaign for him.
And the defender, whose brother Corry, 19, is making a name for himself in the reserves at United, thinks that each high-profile match that passed made him feel more and more at home in Ferguson's team.
"You have your ups and downs in a season but as the season moved on I got more confident in my own ability and settled in quickly I think," he said.
Evans rejects the suggestion that United are now a more close-knit squad following Cristiano Ronaldo's 80-million-pound move to Real Madrid.
But he feels that the Portuguese's departure has meant that United's players have had to step up to make up for the absence of Ronaldo's regular goals.
"I just think that we have to spread the goals a bit and we have been doing that. Goals have been coming all over the pitch but the lads have always had the ability to do that.
"But I think because Ronaldo was such a good player he used to take all the penalties and free-kicks and that is why his name was on the score-sheet a lot."
Owen Hargreaves, meanwhile, is back at United's Carrington training ground full-time after several months in America working to recover from a long-standing knee problem.
Ferguson feels the England midfielder has settled well with his team-mates and hopes he will be involved in the first team within a month.
The United manager said: "He is doing very well and I am very pleased. He is delighted to be back obviously, it has been a long time for the kid but he is back and I think we will get him back to training in 10 days time.
"I think depending on how well he does in training he should be back at the beginning of November."