The 39-year-old has struggled to make a telling impact at Old Trafford this term, reflected in his lack of playing time for the Red Devils.
The former Wales international has only started five Premier League games – lasting 90 minutes in just three of those fixtures – and is yet to find the net.
But Ferguson brushed aside any criticism of Giggs, who featured in a 4-3 victory over Newcastle, although the United boss will not risk the midfielder against West Bromwich Albion.
“How can you a criticise a player who has given you 20 years?” said Ferguson.
“The problem for a player with Ryan’s pride and experience is he probably wants to play more so he loses that game-play time that gives him consistency.
“At his age, I can’t afford to risk that though.
“I don’t think I’ll play him tomorrow simply because of the energy he spent on Wednesday night to help win the match for us. It is simply beyond belief how you can criticise him. I don’t understand it.”
Ferguson hopes to have Danny Welbeck and Ashley Young available for selection against the Baggies after the pair missed the Boxing Day fixture.
“We’re hoping maybe Young will be ready and Daniel Welbeck should be ready,” he added.
“He was ill on Tuesday night but I think he should be okay. At least that’s a couple of bodies back fresh for us. We’ll just assess the rest in terms of tiredness.
“All the players are key now as we’re picking up one or two injuries. Rooney is out for at least another two games. He was a bit better yesterday. We’re going to assess Phil Jones but I don’t think he’ll be ready.”