Kieron Dyer was included in England's final 23-man squad for the World Cup finals on Tuesday, completing an injury-hit midfield including captain David Beckham and Nicky Butt.
Dyer injured his knee while playing in Newcastle United's last game of the season against Southampton on May 11 and is still not fully fit.
"It's a calculated risk, he's got an injury and it's getting better every day. I think the doctors think it's a risk worth taking," his agent Jonathan Barnett told Sky Sports television.
Dyer should play on the right side of midfield where his pace and skill on the ball gives England an extra dimension. Beckham is recovering from a broken bone in his left foot and is hoping to make the first game against Sweden on June 2.
Dyer was initially thought to have no chance of playing in the finals, starting on May 31.
But after showing rapid recovery, Dyer was given until Tuesday, FIFA's deadline for the naming of squads, by England coach Sven-Goran Eriksson to prove his fitness.
FA spokesman Paul Newman said the 23-year-old is "continuing to make progress".
Barnett said: "He's delighted but he's got a long way to go - he's still got to pass the fitness test. It's been touch and go but he's got the time that he's always wanted to be there."
Dyer had a gym work-out on Tuesday to test his fitness and on Wednesday he will have a field session.
Butt has injured his knee but is expected to be fit for June 2. Dyer's potential replacement in the squad, Trevor Sinclair, who played in England's 1-1 draw with South Korea earlier on Tuesday, will now fly home.
Newman said: "Trevor is obviously disappointed at not being in the final 23 but he's very keen to continue his standby role.
"Sven understands and supports his wish to return home and he's specifically praised Trevor for his professionalism, understanding and commitment to Team England."
"Whilst staying on standby he'll train every day at West Ham and we will keep in close touch with him in case anything happens between now and June 1."
Teams are allowed to make a late change to their squad up until 24 hours before their first game provided that the injury would rule the player out of the tournament and is certified by a FIFA physician.