Derby's attackers again grabbed the headlines but Grant's save to stop Nahki Wells putting Huddersfield 2-0 up was a pivotal moment in the match.
Wells had turned in a Danny Ward cross in the 14th minute and pounced on a Richard Keogh error six minutes later to go clear only for Grant to deny him.
That was the turning point as Johnny Russell converted a great pass from George Thorne in the 28th minute before Joel Lynch was sent off for a second yellow card just before half-time.
Huddersfield fell apart with goalkeeper Alex Smithies palming a cross into his own net in the 50th minute and Chris Martin sealed victory from the penalty spot with his 21st goal of the season seven minutes later after Peter Clarke was sent off for bringing down Will Hughes.
It was easy for Derby in the end but their head coach knew how vital Grant's save had been.
"He's got us three points again today because two nil would have been a difficult one to come back from," McClaren admitted.
"I think the work Lee does with Eric Steele, who is one of the best goalkeeping coaches in the country, is taking Lee to another level and that's being proved on the field.
"The goalkeeper has got us another three points on the board and when you are looking at a successful season you are talking about 15 to 18 points that a goalkeeper can get you so Lee is well on target for that."
It was a different story for Smithies but Huddersfield manager Mark Robins had some supportive words for his goalkeeper.
He said: "It's a tough one to take for him and he's disappointed with it but he's a top, top young man and he will bounce back from it.
"He's made a world-class save there at the end from a header and although it doesn't make up for it in his eyes, I think he's been fantastic for us this season and he's only a young goalkeeper.
"He's going to make some mistakes and that won't be the last one he makes that's for sure but he's been in great form for us.
"It was a great test of character for him because the crowd then expect him to drop every one so he had to deal with that and he's come through it pretty well."
Derby can confirm their place in the play-offs with victory at Doncaster on Friday night, but McClaren added: "We can only go and try and win our games and the first job we have to do is make sure we have two feet in the play-offs and keep our focus, that's what we have to concentrate on."