Brady had impressed for the age-group side, scoring seven goals in his 11 appearances and his boss was unsurprised by his call-up to the senior side, recruited as well as Reading centre-back Alex Pearce for the match at Craven Cottage.
King told reporters: "The manager did not just call me last night. Weeks ago he put me on alert that it was a possibility and he continues to do so, which is very good.
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"He rang me again and said he intended to bring Robbie in.
"He's obviously conscious of the fact that we can't qualify for the play-offs now so we're delighted for Robbie and he goes with the congratulations of the squad.
"It's nice as he follows James McCarthy in from this group. That's part of this job – bringing players through. It's a vehicle for bringing players forward.
"Everybody in our dressing room – players and staff - is delighted for Robbie and James before him. We hope that we continue to send players up to the senior squad."
Brady became Ireland's record all-time goalscorer for their Under-21 side with a penalty in their 2-1 loss to Hungary for his seventh strike for the team, and King believes the youngster deserves his senior bow.
He added: "Robbie has deserved his call-up. He's been sensational for us – as the people of Sligo will testify after his previous performances in the campaign.
"He sets the place alive and some of his play has been outrageous.
"He still has lots to learn – he's not the full article and he'd tell you that himself. The players love playing with him.
"He's a great personality. He has a good demeanour and a good manner and he's a credit to his family. I've nothing but good to say about him and I hope he fulfils his potential.
"It's a lot coming at a young age for him but he has big shoulders and I think he’s going to be one of our most exciting players.
"Robbie has the chance to go on and play for Ireland for many years and I hope he does."
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