The Premier League's top teams swarm to Asia chasing the region's growing wealth and unquenchable thirst for English soccer, but they go home counting the fitness costs.

At least two key players - Jan Vertonghen of Spurs and Matija Nastasic of Manchester City - suffered injuries during lucrative friendlies that could see them miss the start of the season, which kicks off in fewer than three weeks.

Manchester United's Wayne Rooney was in Bangkok for less than 24 hours when he sustained a hamstring injury in training and was immediately sent back to Britain for rehabilitation.

He is at least expected back before the start of the season and is penciled in for a return on Aug 6.

Trips to Asia are becoming a hugely important revenue source for clubs keen to cash in on television and sponsorship deals, fees from friendly matches and sales of official merchandise.

But while the clubs' moneymen rub their hands together at the ever-increasing demand for live appearances across the continent, coaches and players alike have admitted the demanding schedule is exhausting.

Spurs manager Andre Villas-Boas was in open revolt after seeing Vertonghen, a first-choice defender, pick up an ankle injury playing on a surface in Hong Kong that his Sunderland counterpart Paolo Di Canio branded "a killer pitch".

"If I can be sincere, I would prefer not to play, but this is the reality that we have to face," the Portuguese said on the eve of his side's friendly against South China.

After seeing his team hammer the hopelessly outclassed local side 6-0, Villas-Boas said he was just relieved to get out without any more injuries.

Hours later, Manchester City manager Manuel Pellegrini found himself in the same boat when he lost first-pick centerhalf Nastasic, also at Hong Kong Stadium, where days of heavy rain had the players slipping and sliding dangerously.

The young Serbian's injury was caused by a kick to the ankle, said Pellegrini, refusing to blame the muddy pitch. Nastasic appeared to be in considerable pain as he was carried from the field in front of a 40,000 sell-out crowd.

Manchester United was the most ambitious team, jet-setting on a whistlestop tour of Thailand, Australia, Japan and Hong Kong. It played five times in just 17 days.

Manager David Moyes said it was inevitable players would pick up minor injuries in such a tight schedule.

"When you've been away for three weeks and played the games in quick succession and had to travel, there's always going to be slight niggles and strains which you want to protect," he said.

His Arsenal counterpart, Arsene Wenger, whose side embarked on a two-week tour that took in Vietnam, Japan and Indonesia - where temperatures nudged above 32 Celsius - also admitted four games in 13 days had taken its toll.

"We are well advanced in our preparation," Wenger said. "It depends now on how quickly we'll recover from that trip because it was very difficult. It will take a few days to recover from that."

England striker Jermain Defoe, who scored a hat-trick in the romp over South China, was another to admit playing on the other side of the world in the fierce summer heat and humidity had been draining.

"A lot of the boys have been really tired, to be honest, as you can imagine with the time difference and training twice a day," he said. "But it's a long season and you need to get fit. It's not a holiday, at the end of the day. We've had six or seven weeks to have our holidays. Now it's time to work."

Liverpool was also in the region, playing friendlies in Indonesia, Australia and Thailand that racked up a total of seven goals, but thankfully for manager Brendan Rodgers there were no significant injury worries.