The Reds were beaten 3-0 at Camp Nou in the first leg of a Champions League semi-final on Wednesday, with a Lionel Messi brace and a Luis Suarez strike sealing a comfortable win for the hosts.
Jurgen Klopp was forced to start Firmino on the bench for the heavyweight clash after he suffered an injury in a 5-0 Premier League victory over Huddersfield the previous Friday.
Wijnaldum was deployed up front his stead, with Mohamed Salah and Sadio Mane completing the front-three, but the trio endured a frustrating night in front of goal.
"After the game, I had way more respect for Roberto Firmino," Wijnaldum said. "I already had it for the way he played as a striker but now I have even more having been in that position.
"I know how hard it is to play well as a number nine for Liverpool, especially if you do it the way he does it and the way we want to play. It's really hard.
"I spoke with him after the game and told him how much respect I had for him playing as a striker for the whole season because it's so tough."
The Reds are still fighting on two trophy fronts, with only a point separating themselves and Manchester City in the Premier League table and the second leg with Barca in Europe to look forward to next week.
However, City will successfully defend the domestic trophy if they win their remaining two games and a turnaround against the La Liga outfit at Anfield may yet prove to great a task.
Wijnaldum confessed that the squad were hugely disappointed with the result in Spain, before insisting there will be an immediate response, adding: "We were really devastated. [But] you always have to regroup, even after winning a game.
"You have to try and do the things you did well again and change the things that you didn't do well."
Next up for Liverpool is a huge trip to Newcastle on Saturday, which they have to win to put pressure on City ahead of their Monday night fixture against Leicester City.
Although he is still hopeful silverware can be salvaged by the end of the campaign, Wijnaldium is confident that if it doesn't, success will be forthcoming for the team over the next few years.
"It's really difficult. The only thing we can do is try to win our games and hope that City slip in one of theirs," he added.
"The focus has to be on the games we played because if we drop points it's over. We have to win and hope they slip.
"The next step [for Liverpool] is, of course, to win something, to be even more consistent than the season before.
"If you look at the seasons so far, we've progressed every season, we're better as a team and more consistent year on year.
"If you can grow like that, in the end, you will achieve something. I hope it will be this season, but if not this season then the seasons after."