The Spaniard looked on as a side featuring 10 changes and including three men who have been told they can leave the club slump to a 3-2 extra-time defeat at St James Park, the first time they have gone out of the League cup in the second round since 1990.
It was the promoted Magpies' third defeat in as many games to date this season and did little to lift the pall hanging over Tyneside after yet another summer of unrest.
However, Benitez said: "We knew that to going into the Premier League, a new division, a new competition, another level, you can win games or you can lose games.
"In this case, we lost this one
It was a game that we were expecting to win - we didn't do it and we have to be ready for the next one.
"I have said that this team will improve and I am convinced that we will do it."
Newcastle got off to the perfect start when Aleksandar Mitrovic fired them into a third-minute lead, but Jason Cummings' double put the visitors in the driving seat until Rolando Aarons struck in first-half stoppage time.
But it was Forest substitute Tyler Walker who headed the Sky Bet Championship side to victory eight minutes into extra-time after Newcastle had dominated the second half, but failed to take any of their chances.
Asked if the result proved that the squad was not strong enough, Benitez, who is still hoping for significant movement both ways during what remains of the summer transfer window, said: "No, it proved that we have to improve in some areas."
Opposite number Mark Warburton was delighted with the efforts of his team, which itself featured six changes.
He said: "For us, it was about coming to an iconic football stadium on a superb playing surface and enjoying it
If you can't come to a place like St James' Park and enjoy a game of football, then something is wrong.
"What we wanted to do was come here tonight and deliver a good performance
We know we're a good team, we know we are getting better, we are gelling as a team.
"We were very respectful - obviously Rafa is a world-class manager with a Premier League squad
We wanted to come here, deliver a performance and leave the field with no regrets and that's what I think we did tonight."
Source : PA
Source: PA