Munroe starred on loan at Holloway's Swindon in the first half of the season on loan from Villa, but made the permanent move to Boro earlier in the January transfer window.
Holloway praised Boro's recruitment team for recognising Munroe's quality, but was astonished Villa opted to cash in on the 20-year-old, who was approaching the end of his contract at Villa Park.
Munroe is still waiting to make his first start on Teesside, but Matt Targett has starred for the Championship promotion hopefuls this season and Boro have extremely high hopes for their new recruit in the future.
RECOMMENDED READING:
Holloway would love to have signed Munroe on a permanent deal but Swindon obviously couldn't compete with Boro. And he can't believe Villa let the youngster go.
The experienced boss says Munroe is one of the best young full-backs he's ever seen, and said Villa have made a "terrible blunder" in letting him go.
"We've just lost the quality he gave us down there," said Holloway.
"I can't get someone from Middlesbrough, who has a four-year contract, who are paying him what they have decided to pay him.
"Well done to Middlesbrough for recognising it. Aston Villa, you should be ashamed of yourselves.
"You let go one of the best future left-backs I have ever seen in my lifetime, and I have seen a few.
"Some of them played in the Premier League with me and for me. I think they have made a terrible blunder."
Earlier in January, before Munroe had completed his Boro move, Holloway said: “First off, I think Aston Villa are wrong, because they're not taking him. That's why this has all happened. They're not giving him a new deal.
“I would strongly shout to everybody that I don't think I've had as good a full-back potentially in my lifetime, wherever I've been, maybe Seamus Coleman borrowing him on loan.
"Maybe he was potentially and I think Fin's in the same bracket, in my opinion. So there you go."