McManus is a member of the Antrim management team, alongside Kevin Ryan and Barry McFall, now preparing for a final against Clare on September 14.
But McManus is unhappy about the scheduling of senior club football championship matches last Saturday, in direct opposition to the U21 hurlers.
McGuinness was named in the Antrim starting line-up but with the Saffrons already struggling for numbers, he was unable to travel to Semple Stadium because of his club Rossa’s SFC tie against St Paul’s.
McManus said: “That’s unfortunate and probably shows you the way that Antrim prepared for this competition [U21 hurling].
“Other counties are together for nine or 10 months but we have had a month of work, not a whole lot more.
“Hopefully this result can go some way towards changing how this is viewed. It shows people that the opportunities are there if they want to put in the work.”
McManus admitted that he didn’t have the full details surrounding the individual case of McGuinness, but revealed: “The club and county couldn’t come to an agreement.
“It’s very unfortunate because the player was the loser in all of it. He’s a good lad and played in both of the Ulster matches. I was very disappointed for him and that decision should never be left to a player.
“There should have been an agreement come to that the player was available to play in both games.
“I’m just not sure about all of the details but the long and short of it is that the player was put in an untenable position.”
McManus also insisted Waterford’s loss is Antrim’s gain, following Kevin Ryan’s reappointment for another year as senior manager.
Despite the vacancy in his native Waterford, Ryan has been installed in Antrim for another year. And he has worked wonders with the U21s after taking over last month before the start of the Ulster championship.
McManus confirmed: “We’re delighted to have Kevin Ryan reappointed. I know that Waterford would love to see him down there. After one year, we already know why. He’s been a massive boost for Antrim hurling and if he gets the required buy-in from clubs and players, the possibility is there for Antrim to progress.”