Doyle made the £6.5million move from Reading during the close season only for a hernia operation to force him to miss the start of the campaign.
But he is now fit and firing on all cylinders and will be looking to hit the target against Pompey for the third successive league match.
Doyle scored the opening goal in the 2-1 win over Fulham at Molineux and also netted in the 5-2 reversal at Sunderland last weekend.
McCarthy said: "Kevin is one of the first names on the team sheet - and quite justifiably so because I think he has been terrific for us.
"He has certainly added that bit of quality. He was our big summer signing and then he gets a hernia which was frustrating for him.
"Having him available has made a big difference to us and I have absolutely no worries about the fee we paid.
"It was money well spent and he will only get better. Whatever value he was will increase with his performances."
Saturday's opponents Portsmouth have lost their opening seven league games, the worst top-flight start by a team in 79 years, but McCarthy is guarding against any complacency, particularly among the Wolves fans.
And he quickly played down the plaudits his team received for their general performance at Sunderland despite the final scoreline.
He said: "I think it is the most dangerous game of the season. We should all be coming here on Saturday with the attitude that it is going to be a tough game.
"We should have the attitude, 'the Pompey fans are great but we are going to out-shout them.'
"We will have to play at very best level to beat Portsmouth. Anything less than that, and there is a danger it could be the wrong result.
"They were very unlucky against Everton. There were two teams last week who came away thinking 'how did we lose?' We were one and the other was Portsmouth.
"But, when you take away the trimmings, take the cards and mistletoe down, see the turkey off, we actually got beat 5-2. That is the cold reality of it.
"We got zero, zilch for our performance. We gave goals away and missed opportunities ourselves.
"I was pleased with how we played but we lost. For all the plaudits we got for good spirit, good energy, good football, we still got beat. I'd sooner people say, 'You were hopeless today and you won 1-0."'
McCarthy knows his Pompey counterpart Paul Hart will not want sympathy from him but believes he could use the club's current predicament and uncertainty to galvanise his players.
He said: "I'd sooner have a bit of assistance from someone rather than a whole load of sympathy. He will not be wanting it from me and he will know full well I won't be giving it to him.
"I can empathise with him. He takes the job, he keeps them up, he did well, and then they sell a whole clatter of his best players.
"He has replaced some with loans and, how he is expected to be just as good a manager, I don't know. You are determined by your players.
"But, with the problems they are having, all those things I would be using if I was Paul Hart to motivate them.
"When you get written off, and if things at the club are not right, if you can just create that backs-against-the-wall feeling and say 'sod everyone else, it's all about us' then they can become a very dangerous opponent."