The QPR centre-half was twice at fault for goals on Monday night as a 3-2 defeat at Craven Cottage left Redknapp's side seven points from safety with as many games to play.
Samba became involved in an altercation with some of his Twitter followers in the hours after the game but later said he wanted to "move on" and focus on QPR's fight for survival.
And Redknapp has no concerns about a player who cost £12m when he arrived in January.
He said: "Chris had a nightmare against Fulham, but he's been fantastic for us. He's a top player. He could play for anyone in the top four.
"He's not the first to make a mistake and he won't be the last. He'll come back from it, though, and we'll see that on Sunday."
Defeat at the weekend would be a third on the spin and leave QPR 10 points behind a Wigan side again threatening to escape trouble late in the season.
Redknapp said: "It's a big game for us. All we can think about is winning the game. We need to win. Nothing else will be enough.
"I've been pleased with the way we've been playing lately. We look like we're going to score goals now. We've just got to cut out the errors. We will never give up. We've got to keep going and that's what we'll do.
"If I stop believing, everyone will. I don't feel we've had it. I still believe we can do it, I really do."
Redknapp will be without Shaun Wright-Phillips after an ankle operation ruled out the winger until next season.