Romo, who missed last Sunday's 28-17 loss to the Arizona Cardinals with his third back injury in the last 18 months, arrived with the team on Tuesday but skipped Wednesday's practice.
Dallas coach Jason Garrett says Romo is still not assured of starting on Sunday, when the Cowboys (6-3) play the Jacksonville Jaguars (1-8) at Wembley Stadium.
"He has to be functional," Garrett said. "He has to be able to protect himself."
Romo has fractures in two small bones in his back. The injury is unrelated to a herniated disk sustained last year and to off-season surgery to remove a cyst earlier in 2013.
Instead of Romo, backup Brandon Weeden played against the Cardinals, throwing two interceptions and one late touchdown.
Cowboys owner Jerry Jones watched practice Thursday and said he expected Romo to play.
"I'm anticipating him playing," Jones said, noting that Romo had a comfortable flight to London. "Have no reason to think that he won't."
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