Ian Baraclough admitted to an overriding feeling of frustration after Thursday's Nations League defeat to Greece.
Northern Ireland kicked off a new campaign with a depressing 1-0 loss at Windsor Park.
Tasos Bakasetas' cool finish in the 39th minute sealed the win for Gus Poyet's Greece at Windsor.
Read more: Northern Ireland vs Greece as it happened in the Nations League
It was the defining moment in a disjointed game in Belfast, one which Northern Ireland failed to threaten in.
Northern Ireland are still waiting on their first win in the Nations League, having picked up just two draws and nine defeats in their 11 games so far.
"It was frustrating," Baraclough told Premier Sports.
"I think we looked like a team that hasn't played in a little while. In the second half we upped the tempo and showed more bravery on the ball and pushed them back.
"We lost a goal from a breakaway, really, missing a challenge in the middle of the park. And that put us on the back foot."
He added: "We talked about intensity and setting the tone from the off, but both teams were allowed to play to their own tempo. We pressed at times and other times we weren't sure.
"That can happen when you have new players coming in, and who haven't played for a while domestically."
Next up is a trip to Cyprus on Sunday, with Northern Ireland swapping the rain of Belfast for the hot and humid conditions of the AEK Arena in Larnaca.
"We will look for freshness and see how we recover after travelling to Cyprus tomorrow. It is also a warmer country and that will take it out of us," Baraclough said.
"But we knew that and have tried to prepare for that as much as possible. And we look forward to the game.
"We knew Greece would be a tough nut to crack. They have rebuilt as well and have played together as a group for the last couple of years.
"And that showed.
"But second half I was pleased how we reacted the changes we made had an impact."
Niall McGinn believes a lack of recent competitive football for many players resulted in a jaded performance against Greece.
The Dundee winger said: "It is sheer disappointment straight after the game.
"At the start of any Nations League campaign you want to get off to a positive start, and I think all in all we didn't test their keeper enough.
"It was a difficult 90 minutes where a lot of the boys looked like they hadn't played in four or five weeks.
"I think that showed in our display.
"But when we switched to the 4-3-3 in the second half we created a few more chances and tried to et balls into the box.
"But all in all it's disappointing. Greece got their lead and defended it well.
"It was frustrating we couldn't get the goal. There weren't many chances, and you just hoped if you got one then you'd put it away.
"The good thing is we have three more games coming up so we have to dust ourselves down and go again."
One of the positives from Thursday's Nations League defeat was the senior debut of Manchester City teenager Shea Charles, while Liverpool youngster Conor Bradley started the game.
"You saw young Shea come on and make his debut. It is a big learning curve for them and there are a lot of experienced boys here who play at a high level, such as Jonny (Evans) and Davo (Steven Davis). So they are learning off the best," McGinn said.
"We have to bring them along and make sure they take their opportunities when they get them.
"We now go to Cyprus and try and get a positive result. The conditions will be tough out there. A lot of boys have had holidays recently so I am sure they will be used to it.
"But we have to make sure we use possession as best we can and take our chances."
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