Rooney has been troubled by an ankle and groin injuries in recent weeks, the latter of which forced him off early in Manchester United's final Premier League encounter with Stoke last weekend.
That match was Rooney's 49th for club and country in a season when the 24-year-old has shouldered United's hopes like no other time during his Old Trafford career.
And Donovan, who will lead the United States against England in their World Cup opener in Rustenburg on June 12, believes Rooney is showing signs of being "worn out" after his heavy workload.
"Rooney is a top player. In my opinion, he is one of the best players in the world right now," Donovan told Yahoo! Sports.
"But the other part of it is that he has played a lot of games and he is tired. He is probably worn out.
"Injuries are a part of that. Things happen when players are tired. When they just keep adding games to the schedule, it makes it difficult.
"Any time you come into a World Cup, because of the timing of it, big players who play a lot of games in Europe are coming right at the end of their season, so it is always an interesting dynamic how to balance that."
Donovan saw the pressure on Rooney and his England team-mates first hand last season when he spent a part of the campaign on loan with Everton.
The 28-year-old made 10 appearances for the Toffees as he sought to maintain his match fitness ahead of this summer's finals and after sampling the football-centric culture in England believes all the pressure will be on the USA's Group C rivals in South Africa.
"Of course, there is more pressure on the English players," he added.
"If we don't do well in the World Cup, people care for a little bit. But if England doesn't do well in the World Cup, it is absolutely devastating to their country, their families, to their people.
"That is something they [England] live with. I don't know if they look at it that way, but that is how I look at it."