The 26-year-old is set for an extended run in the side after Edwin van der Sar's injury and endured a difficult afternoon at Wembley.
Foster's kicking looked suspect and he could have done better for both of Chelsea's goals, as Carlo Ancelotti's side ran out winners on penalties.
But Ferguson insists he has "no worries" over Foster's capabilities. "Knowing Ben and the ability which he has, he should have done better with the second goal,'' said the United boss.
"However, it was his first big challenge. He has waited a long time, has had his injuries and in pre-season I spread the load between the three keepers, and maybe we did not have enough minutes for him.
"But the important thing is, knowing his ability, it means I do not have any worries about him.''
Ferguson could find no words of comfort for referee Chris Foy however, after the incident on 70 minutes that changed the game.
Substitute Michael Ballack appeared to elbow Patrice Evra as the full-back charged down the left flank, but with the United player down, Foy allowed play to continue as Chelsea broke quickly, with Frank Lampard's shot beating Foster inside the far post.
Although the Premier League champions - who took the lead through Nani, later to go off with an injured shoulder - forced a penalty shoot-out when Wayne Rooney slotted home in stoppage time, Chelsea keeper Petr Cech was the hero with two saves.
Ferguson, however, believes Foy should have intervened when Evra was left flat out on the Wembley turf.
"I think the referee has made a rod for his own back,'' he said. "He stopped the game twice, first when Nani was down, then on the second occasion Ballack has gone down, which we have seen before with him.
"I asked the referee, he said he thought it was a serious incident, and he thought the player needed treatment, whereas he [Ballack] was up right away. Once he has done that, I do not understand why he has not stopped the game when Evra is lying down.
"With a replay you can see Ballack has elbowed the boy, and if the referee had seen it properly, it is a red card. The referee is in line with it, and should have at least stopped the game, but he did not.
"Of course, then they went on and scored the second goal. It is hard to blame the Chelsea players because it is a professional game. But it is making it very difficult for referees [to know] when players are acting to get the game stopped.
"It is an area I am concerned about and we have got to find a solution.''
Portugal midfielder Nani impressed, but Ferguson revealed he could now be set for a spell on the sidelines.
"He had done very well and we are very pleased with his performance.''
Ferguson felt United should have put the game beyond Chelsea following their dominant first half, with defender Ricardo Carvalho eventually pulling the FA Cup winners back in it on 52 minutes.
"We should really have put the game to bed,'' he said. "When you get chances in big matches and do not take them, you can suffer and we did.''