Smalling, 20, started his fifth Champions League game of the season on Wednesday night and produced another outstanding display in United's otherwise disappointing goalless draw at the Stade Velodrome.
How long Smalling retains his spot depends on Ferdinand's recovery from the calf injury he suffered in the warm-up ahead of his club's shock defeat at Wolves at the beginning of the month.
The former Fulham player is not conceited enough to believe he can keep the England captain out of Sir Alex Ferguson's first-choice starting line-up at present. However, he is certainly providing the United boss with concrete evidence that a replacement is at hand whenever Ferdinand's body lets him down.
"When you get the shirt and a position in the team you don't want to give it up,'' he said. "I understand that Rio is the main man. But when he got injured it was time for me to step up.
"They are big shoes to fill. Hopefully through my performances and the sort of games and massive occasions I have been playing in, I repay the manager's faith and justify him playing me.
"I know I will have to bide my time but I am always going to be ready because no player wants to be sitting on the bench.''
United now face a series of hugely significant matches on the domestic front. In pure eye-catching terms, Saturday's trip to Wigan is bottom of the list.
However, after Arsenal's victory over Stoke it is a chance to re-establish a four-point lead at the Premier League summit while the Gunners are otherwise engaged looking to secure their first silverware in six years when they take on Birmingham in the Carling Cup final.
And after that come the heavyweight clashes against Chelsea on March 1 and Liverpool on March 6 before, almost certainly, a visit from Arsenal in the FA Cup sixth round six days later.
Rather than be daunted by it, the fixture list is one Smalling relishes. And he cannot wait to get stuck in.
"I didn't expect to get games of this nature yet," he said. "I had the Manchester derby a couple of weeks ago and now Marseille, which was a totally different experience. I heard a lot about the stadium. You don't often get 90% of the fans booing you just doing your warm up before the game.
"We are at a massive stage of the season now, with so many big games coming up. But they are matches you should relish. I am really looking forward to the run-in.''