Ireland made light work of USA at Aviva Stadium on Tuesday, with a second-half brace from Robert Brady inspiring the lopsided international friendly win.
Anthony Pilkington and James McClean were also on target for the home side in Dublin.
And German boss Klinsmann was far from impressed with how USA performed as they crashed to back-to-back losses following Friday's 2-1 defeat to Colombia in London.
"I got in trouble a couple of years ago for saying it, but I'm telling you again: we have to get nastier," said Klinsmann.
USA's poor post-World Cup form continued, with the Americans without a win in four matches since edging Czech Republic 1-0 in a September friendly.
Klinsmann has blooded a number of youngsters into the team, exposing them to the rigours of international football.
That was the case again in Dublin, with six players with less than 10 caps featuring against the Irish - Bill Hamid (two), Alfredo Morales (six), Bobby Wood (five), Greg Garza (five), Rubio Rubin (two) and Jordan Morris (one), who made his international debut.
With that in mind, Klinsmann warned USA fans to expect more pain along the way as the squad undergoes some changes.
"We saw a lot of things: some good stuff, some not so good stuff so we will address that obviously with the guys. It finished off a long year in 2014, which overall was a positive year and we are building," said Klinsmann.
"When you build a new cycle and introduce new, fresh players into the group then there will be some growing pain along the way – that is just normal.
"Obviously you don't want to lose games, absolutely not, but the development of those players does not come without pain."
