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Keogh was one of four new faces at Molineux mid-season - along with Michael Kightly, Stephen Ward and Michael McIndoe - which McCarthy believes galvanised his side.
"The £600,000 we spent on Keogh looks like fantastic business to me," said McCarthy.
"They have improved us there is no doubt about that, and they have played a big part in getting us where we are.
"Everybody always looks for reasons why they have settled so well but let's see how they - and we as a team - handle the next 12 days."
If Wolves win promotion to the Premiership while Keogh is a player with them, Scunthorpe will receive an extra £250,000 - with a percentage of that owed in turn to Leeds United -as part of the deal that saw him exit North Lincolnshire.
Keogh was instrumental in Wolves' 4-1 win at Leicester on Sunday which sealed their play-off spot.
He converted a penalty-kick four minutes from time, ending a hoodoo which had seen his side miss four times from spot-kicks this season.
"I wasn't really designated as the taker - I just picked the ball up and said I'm having it," admitted the striker.
"I should have had a goal earlier on when I hit the woodwork, so wanted to make amends.
"But there's still a job to do, and everyone has come back into training focused on that."