It was a scary night for David Beckham and for U.S. fans awaiting his move to Los Angeles: The soccer star suffered a knee injury Sunday night during his match in Madrid.

Beckham put in a spirited performance for Spanish team Real Madrid but strained his right leg after reaching for the ball and crashing into advertising signs near the opposition goal. The injury occurred during the second half of his team's 1-1 draw against Getafe.

"It looks serious but I do not want to say how bad until the doctors do the scan," Real Madrid coach Fabio Capello said after the match.

After the injury, Beckham, 31, tried to play on but limped off in the 63rd-minute. He was later diagnosed as damaging the lateral internal ligament of his right leg and is undergoing further tests Monday.

The injury could impact plans for the soccer star, his wife Victoria and their three children to move to America later this year.

"The day started so well with a birthday party for his son," a family friend tells PEOPLE. "But it ended with medical scans." (Beckham's eldest son, Brooklyn, turned 8 years old on Sunday.)

"Both David and Victoria are really anxious at the moment, waiting to hear what the results will be," adds the friend. "It's not a good day for the family."

In January, Beckham inked a five-year deal with L.A. Galaxy that is worth up to $250 million with endorsements. Last week it was announced that Victoria Beckham, 32, would star in a six-part reality show on NBC about her move to America.

Before the match on Sunday, the couple had taken sons Brooklyn, Romeo, 4, and Cruz, 2 to a Madrid burger restaurant. "The kids had a ball. It was Brooklyn's eighth birthday party," says the friend. "And all his school friends were there too."

Beckham's hopes of rejoining the English national team this year were also dashed after the injury on Sunday night. Beckham had played well, even firing one shot into the opposition's goal crossbar.

"It's difficult to know what is going to happen now," says the Beckham friend. "Everyone is praying this is not major."

Beckham's rep could not be reached for comment.