Emmanuel Riviere will miss the start of Newcastle United’s season.
The striker – who had been with his team-mates in the USA – has returned to England to have surgery on a knee problem.
And Riviere, along with midfielder Remy Cabella, won’t be available for the club’s season-opener on August 9.
Cabella did not travel to the USA after suffering a thigh injury in training.
Asked about Riviere, McClaren said: “He’s been sent back home.
“He’s had a scan on a knee problem, which needs a small operation, so he’s back home to get that done.”
Riviere’s injury leaves McClaren short of options up front, though Anderlecht striker Aleksander Mitrovic is close to joining the club.
Meanwhile, it has been reported in France that Cabella faces between six and eight weeks on the sidelines.
McClaren, however, is reluctant to put a timescale on his return.
“We don’t know how long it is exactly, but before he came away, he felt a little something in his thigh,” said McClaren.
“We just have to see how long that will be.”
Defender Curtis Good is sidelined with a hip injury, and won’t be available for tomorrow night’s game against Portland Timbers.
Youngster Adam Armstrong missed Saturday’s 1-0 victory against Sacramento Republic, but the teenage forward could be available at Providence Park.
McClaren said: “We might see Arma in Portland, but definitely not Curtis.”