United States coach Jurgen Klinsmann says he is concerned that "a lot" of the players on his World Cup squad have played poorly since this summer's tournament in Brazil.

Klinsmann has repeatedly encouraged his players to join clubs at the top level, even saying that those moving to MLS must prove themselves again with the national team.

On Monday, Klinsmann turned his attention to the Americans playing their club soccer in Europe, warning against complacency and urging them to find opportunities to stay in the starting lineup.

"Right now at the end of 2014, going into the last two friendly games, looking at the last couple months, is a bit of a concern to us," Klinsmann said in a video released by US Soccer.

"A lot of our players that played really well in the World Cup and were consistent and worked very, very hard really dropped their performances after the World Cup, even after they are starters with their teams or in many cases unfortunately are not starters in their teams anymore. And I'm looking more at the European players in that moment."

Sunderland's Jozy Altidore has only been used as a substitute in the Premier League this season. AP Photo/J Pat Carter

Jozy Altidore remains the Americans' first-choice striker, but he has yet to start for Sunderland in the Premier League this season, and has scored just once in the Capital One Cup.

Geoff Cameron only just moved into Stoke's starting XI in recent weeks after teammate Glenn Whelan broke his leg, while Alejandro Bedoya has lost his starting spot at French club Nantes.

In Germany, 19-year-old Julian Green has only made three appearances while on loan at Hamburg, while John Brooks lost his spot with Hertha Berlin midway through September. At Borussia Monchengladbach, Fabian Johnson is regularly used as a substitute while dealing with an Achilles problem.

Klinsmann said his players need to be at the top of their games at all times, not just in the lead-up to a World Cup.

"This is something that we have to improve over the next years," Klinsmann said. "Our players have to become stronger to kind of deal with the big moments and then going back into their season and making sure that they play in their club teams on the highest level and especially as starters."

The U.S. finish the year with two friendlies later this month -- Colombia in London on Nov. 14, followed four days later by a match against Republic of Ireland in Dublin -- and Klinsmann said he would address his concerns with his European-based players at that time.

"This is going to be a huge topic when we communicate with the players now going into these two games," Klinsmann said. "Talking to them and telling them, 'Listen, it takes more to become consistent. It takes more professionalism and strength to come after World Cup and show those performances again in your club level.'"