Republic of Ireland boss Martin O'Neill said Wales punished his team's long list of absentees in the thumping 4-1 Nations League win in Cardiff.
James McClean, Shane Long and Robbie Brady were among those out injured while Declan Rice and Harry Arter had declared themselves unavailable.
"We're unable to be missing five or six Premier League players and still put on a real show," O'Neill told Sky Sports.
"We were well beaten. It was difficult. We were missing some key players."
The Republic had not lost to Wales since 1992 but Tom Lawrence's goal and a superb strike by Real Madrid star Gareth Bale put the home side 2-0 up inside 18 minutes in the competition opener.
O'Neill says early goals set Republic back
Outstanding Chelsea 17-year-old Ethan Ampadu robbed Jonathan Walters of possession to set up Aaron Ramsey's third Wales goal before half-time with Connor Roberts making it 4-0 prior to Shaun Williams' Republic reply.
"They scored very early in the game. Their second goal from a world-class player [Bale] was a world-class goal. That sets you back. Two-nil down after 15 minutes. It's a tough way back. It was a hard evening," continued O'Neill.
"We need to be at full strength if we can be. I'm not so certain the last time we were at full strength but I've not known when we've been as decimated as this."
The under-strength Republic team's dismal display was a far cry from the performance 11 months ago when McClean's goal earned a thrilling 1-0 World Cup qualifier win in Cardiff.
Republic 'not brave enough' on the ball
Former Manchester United star Ryan Giggs has since taken over from Chris Coleman as Wales manager and his young team produced some thrilling football in Thursday's game.
"There's a new manager who has come in and he's blooding some young players," added O'Neill.
"They look as if they have some good players but we had a different side out than when we won the game this time last year."
Both O'Neill and his captain Seamus Coleman felt that the Republic players had not been "brave enough" on the ball.
"All round, Wales were better than us tonight. Individually, we were poor," said Coleman, making his return to competitive internationals for the first time since suffering a double leg break in the World Cup qualifier against Wales 18 months ago.
"We didn't give each other enough options on the ball, which is important.
"Any time you put on this green shirt, it's an honour and you've got to play with that passion, pride and bravery on the ball, and we didn't do that."
The Republic face Poland in a friendly in Warsaw on Tuesday before resuming their inaugural Nations League campaign next month with home games against Denmark and Wales.