Liverpool manager Brendan Rodgers says fatigue is to blame for teenage winger Raheem Sterling losing his "zip".
Sterling, 18, established himself in Rodgers' team after making his Premier League debut for the Reds in August.
He earned his first England cap in November before signing a new five-year contract at Anfield, but has only started two top-flight games in 2013.
"That zip has gone and it's down to fatigue. Was that to be expected? Yes it was," said Rodgers.
Former Queens Park Rangers trainee Sterling, who moved to the Merseyside club in February 2010 aged 15, had not started a senior match for the Reds before this season.
But he forced his way into Rodgers' first-team plans and responded with a series of impressive displays.
It earned the Jamaica-born teenager an international call-up from England coach Roy Hodgson, who described Sterling's rise as "meteoric", before starting him in the 4-2 friendly defeat in Sweden.
However, Rodgers has taken the winger out of the limelight in recent weeks after expressing concerns about mental and physical burnout.
"This is a kid we're talking about, so we have to take a wee step back," said the Northern Irishman.
"It is so mentally and physically draining, this league. It hurts experienced professionals and this boy has stepped up from the academy, played football and earned a contract off that.
"We have to nurture that and, at times, he has to have the mentality to keep learning.
"If he didn't kick another ball this season, he's had a good one.
"But what you are seeing now is more tiredness of the legs when before he could go up to people and take them on."