Sterling had a patchy season with Manchester City but won a big vote of confidence from England boss Roy Hodgson when he was named in the starting XI for the first two games of Euro 2016.
In both games his electric pace was not matched by composure on the ball and his carelessness in possession saw him hooked off at half-time against Wales before being jettisoned completely against Slovakia.
He has also been jeered by Three Lions fans and there have been reports that incoming City manager Pep Guardiola called the winger personally to raise his spirits.
But Rooney insisted it has been business as usual for the 21-year-old around England's Chantilly base ahead of Monday's last-16 meeting with Iceland in Nice.
Remembering 'beautiful' and 'amazing' Blake and Tristan Barrass three years on from tragic Sheffield brothers' funeral
Sheffield Council employees sacked as fraud increases including theft and excessive internet use during work
EuroMillions: Mystery winner from South Yorkshire has won £79,242.50 on The National Lottery
Power cut Sheffield: Hundreds of homes in Hillfoot area without power ahead of 28C heatwave today
Sheffield Council tenant forced to pay £5,000 and struck from housing register for fraud
"For me, what I've seen, his state of mind is fine," said the captain.
"He's been normal around the camp. What we've seen on the training pitch from Raheem, he's been fantastic: sharp, taking players on, scoring goals.
"There's no issue with Raheem at all. His attitude isn't questioned.
"He stays behind after training doing what he does normally. He's a huge asset to us as a team, a fantastic player and one who can turn the game in a split-second. If anyone is questioning his attitude or state of mind, they'd be mistaken in having that judgement."
England are heavy favourites to reach the quarter-finals, where hosts France await, given the relative richness of their resources.
Iceland's population is 165 times smaller and their FIFA ranking 23 places lower, but Rooney would not downplay the task ahead.
"We're not going into this game showing a lack of respect for Iceland. We're showing full respect," he said.
"The size of the country is just a number. They put the same number of players on the pitch as us, so it's a fair contest in that respect."