Rooney is out of tomorrow's Champions League clash at Valencia.
And midfielder Gibson told the Daily Mail: "I think it's just a slight confidence issue from the World Cup that he's brought back with him.
"Although he's been around a long time he's still a young player at 24. I think with young players form comes and goes sometimes and he's just having a bad patch.
"I don't know why he's stopped scoring now but, hopefully, he will because, if we're going to win the league, we're going to need him to start again. Fingers crossed he does start."