Valencia, who missed United's four-game tour of the Far East due to an extended break following World Cup duty with Ecuador at the end of last season, has linked up with Ferguson's squad in Germany in the build-up to the four-team Audi Cup in the Allianz Arena.
The 23-year-old arrival will be expected to fill the right wing position at United in the wake of Ronaldo's £80m transfer to Real Madrid.
But with Ronaldo scoring 68 goals for United during his last two campaigns at the club, Ferguson concedes that becoming a direct replacement for the Portuguese will not be easy for Valencia.
He said: "I hope we have found the answer to Cristiano, but he scored 26 goals last season and that, for a winger, was exceptional.
"He was fantastic for us, the best player in the world, and we did very well to keep him for six years.
"With foreign players, it is always difficult to keep hold of them. Cristiano wanted his move and I hope does well. I'm sure he will do.
"Antonio is a player that we watched for some time last season. He is young, athletic and has good power and, at 23, we think he can develop with us.
"He has potential, though, and we believe he can develop and he will start against Boca.
"Fitness-wise, he is still behind the rest of squad, but he has spent some time at Carrington while the rest of the squad was away in the Far East."
Nemanja Vidic and Northern Irish defender Craig Cathcart have joined the United squad in Munich, but England midfielder Owen Hargreaves has missed out on the opportunity to return to Germany, having launched his career with Bayern.
Hargreaves had been due to link up with the squad in Munich to begin light training following two knee operations last December and January.
But Ferguson has admitted that the player, due to return to action in September, remains in the USA to continue his recovery at the Colorado clinic of knee surgeon Richard Steadman.
Ferguson said: "Owen is still in the United States, continuing his rehab.
"We hoped that he would be here with us, but he has had a major operation and there is no point rushing him.
"He is still young and he has plenty of time on his side, so he won't be rushed."