De Gea has made several high profile errors over the past 12 months.
"I don't see David's decline as big as what people have been making out," Giggs told Premier League Productions.
"I think he's still a top goalkeeper. I think the introduction of Alisson and Ederson as well as the form of Man City and Liverpool hasn't helped him but he's a top goalkeeper.
"I've played with him and I've coached him, he's a brilliant goalkeeper but he has made a few more mistakes than he has done in the previous five or six years.
"I think he's had a very unsettled defence in front of him and that hasn't helped him; that wouldn't help any goalkeeper no matter how good you are.
"He's a top, top goalkeeper. When you start messing about in the goalkeeper position as we found out when Peter [Schmeichel] left it can be tricky. I don't think he's at a stage where you need to think about changing him."