Fernando Torres will miss Liverpool's next three matches after a scan revealed he has a hamstring injury that will sideline him for 10-15 days.
The Spain striker suffered the injury in Wednesday's 2-1 win over Belgium, limping off after 16 minutes.
Torres, 24, will miss Premier League matches against Wigan and Chelsea either side of a Champions League game at his former club Atletico Madrid.
"I am very disappointed about this," said Benitez on Thursday.
He also hinted he would want answers from the Spanish authorities about the injuries Torres has picked up on international duty.
Benitez said: "With us, Fernando has had one hamstring injury in 18 months. We need to check why there is a difference with Spain.
"For me the reason is too many games. He played two games in five days, one of them against a country who are not of the top standard. It can be avoided, maybe a better selection.
"Fernando wanted to play next week in Madrid, which will now not happen. His injury is probably the worst he could have had. It is a very bad situation for us, and it is not the first time it has happened."
Torres, Liverpool's top scorer last season in his first year with the club, may even miss a fourth game - the Premier League match against Portsmouth at Anfield on 29 October.
"There are too many international games, and the demands on the players are too much"
The Spaniard has struggled with hamstring problems before, most recently at the beginning of September when he was sidelined for the international break after tearing the muscle in the goalless draw against Aston Villa.
Another Liverpool forward, Ryan Babel, damaged his ankle playing for the Netherlands in their 1-0 win in Norway.
Babel, who missed Euro 2008 with ankle ligament problems, played no further part in Wednesday's match after he went down under a tackle from Blackburn's Morten Gamst Pedersen on 26 minutes.
With Liverpool, second in the Premier League, facing a number of important games in the coming weeks, Benitez is disappointed to lose key players and has voiced his discontent at the international schedule.
"To lose any players is a big blow," he said.
"There are too many international games, and the demands on the players are too much. Someone needs to analyse this situation, because it is putting the players at risk of injury."
606: DEBATE
Liverpool midfielder and fellow Spaniard Xabi Alonso told the club website that Torres' injury was a blow: "It is undoubtedly bad news.
"We have to wait and see what the tests say, but we do not stop now until around Christmas, and we have a big calendar coming up - we cannot let up.
"It is not the first time this has happened [to Torres] with the national side, and it is a blow to have an injury when he is in such a good streak of form."
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