Jose Mourinho is "totally against" international friendlies after Chris Smalling and Phil Jones picked up injuries on England duty.
Jones was forced to leave Gareth Southgate's squad with a toe injury suffered in training after a challenge by Smalling, who pulled out himself with a leg problem after starting England's friendly defeat in Germany.
Both defenders missed England's 2-0 World Cup qualification win over Lithuania, leading Mourinho to question the need for mid-season friendlies.
"I am totally against the friendly matches," he told Soccer Saturday. "I think friendly matches for the national team only make sense before the final phases.
"A couple of weeks before the Euros or a couple of weeks before the World Cup makes sense. But mid-season friendly matches mixed with qualification matches, I don't think that makes sense.
"On top of that the matches are not really big matches so I am not a big fan. But I think one day I will be there so I cannot be very critical."
United had 13 players on international duty over the past week, with Antonio Valencia, Marcos Rojo and Sergio Romero making the long trip to South America.
Mourinho's side face West Brom when the Premier League resumes on Saturday and the United boss believes the hectic schedule leaves his side at a disadvantage.
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"I don't know how but both central defenders got injured in the English camp even before the match," Mourinho added.
"The match was on the Sunday against Lithuania and they were injured in the training session. We lost four, I think [Paul] Pogba is also out, that's five, so we are a bit in trouble.
"I watched Valencia's match yesterday in Ecuador, Marcos Rojo in Bolivia, but they don't arrive until Thursday afternoon.
"If you compare our situation with our opponents, West Bromwich Albion, only one player on international duty. They had time to work, time to rest and prepare. It's going to be hard."
Watch the full interview with Jose Mourinho on Soccer Saturday from 12noon on Sky Sports News HQ