LONDON – Amir Johnson didn't participate in the 76ers' practice or team photo Wednesday, because the reserve big man became ill after arriving here a day earlier.

Richaun Holmes will back up Joel Embiid if Johnson is unable to play in the NBA London matchup against the Boston Celtics at 3 p.m. Thursday at The 02 Arena.

Johnson, 30, has averaged 4.8 points, 4.8 rebounds, 1.5 assists and 15.6 minutes in 36 appearances. He has started eight games.