Ralf Schumacher's Formula One title hopes disappeared on Saturday when the German was ruled out of Sunday's Italian Grand Prix on medical grounds following a heavy crash last week.
Williams, eight points ahead of champions Ferrari at the top of the constructors' standings, said Spanish test driver Marc Gene will race instead after the stewards agreed to the substitution.
"Of course, it's the end of Ralf's title hopes but for the team it should be okay," said BMW motorsport director Mario Theissen.
The German was fourth overall, 14 points behind his pacesetting older brother and five-times champion Michael in a Ferrari. There are just two races remaining after Monza.
"Ralf reported a headache earlier this morning and went to see [FIA medical delegate] Sid Watkins to explain it to him, and said he was not sure he can drive," said Theissen.
"So Sid said it would be the right decision not to drive. Maybe he even said he must not drive.
"Immediately we put Marc in the car, changed everything. It took about an hour to an hour and a half to change the settings for Marc and I'm sure he will do a good job."
Ralf took part in free practice on Friday, but failed to set a time in first qualifying after running wide and missing part of the chicane.
Williams said in a statement that Ralf did not feel 100 per cent fit and had decided with the team and Watkins not to race.
GENE'S CHANCE
Gene last raced for Minardi in the 2000 Malaysia Grand Prix. He scored one point in 33 races for that team and has twice before raced at Monza.
For the first time in his career, he will start in a winning car. Colombian team mate Juan Pablo Montoya, like Ralf, has won two races this year and is chasing the title a point adrift of Schumacher.
Gene attends all the Grands Prix, where he acts as a team speaker at functions for guests, and filled in for Ralf at the Monza tests after the German crashed on September 2.
"I think Marc has done a lot of everything, so he should be okay," said Theissen.
Ralf spent a week resting at his home near Salzburg and reported fit at Monza after passing an extensive medical in Austria on Wednesday.
Friday was his first day back in the car since his crash, however, and Monza is the fastest circuit in Formula One.
"Ralf...will now rest for a few days more to recuperate completely in order to return to the cockpit for the U.S. Grand Prix in Indianapolis in two weeks' time," Williams added in a statement.
Williams chief operations engineer Sam Michael said Gene would have no problem during the race, but would have to come to terms with the new single-lap qualifying format that has been introduced since he last competed.
"Now we have to make sure we get the best position for Marc to help us with the constructors' championship," he said.
Ralf Schumacher won this year's European Grand Prix at the Nuerburgring and in France and was considered a championship favourite earlier in the year.
