The McLaren driver will miss Sunday's Bahrain Grand Prix after sustaining a hairline crack in his shoulder while playing tennis.
The Spaniard will replace the Colombian at McLaren for the rest of the season.
The Colombian will race for McLaren in 2005 after completing his contract with Williams.
The Colombian will not be allowed to compete in NASCAR until his McLaren contract ends, team boss Ron Dennis said.
It was the first time since 1994, when Schumacher took his first title at Benetton, that the award had not gone to the same year's Formula One champion.
It was the second win of Montoya's 41 race career and the first of the season for BMW-powered Williams
Juan Pablo Montoya accused Rubens Barrichello of illegally blocking him in order to help Ferrari team mate Michael Schumacher.
Juan Pablo Montoya spun out team mate Scott Pruett late in the race before claiming his first Nascar victory.
The McLaren driver will leave Formula One at the end of this season to drive in NASCAR.
The defending F1 champion expects the title to be decided by a last-race duel between himself and the Colombian.
McLaren's Juan Pablo Montoya, returning to Formula One after missing two races due to injury, crashed heavily in practice.
Montoya was black-flagged after 57 of the 73 laps and retired in the pits while third.
On a pivotal moment on lap two, he was given a 'drive-through' penalty after an incident with Ferrari's Rubens Barrichello.
The Colombian had a blistering late lap to outpace his German team mate at Williams Ralf Schumacher.
Renault's Fernando Alonso took a giant step towards becoming Formula One's youngest champion by finishing a close second.
The McLaren driver will make his return in Sunday's race, subject to final medical approval on Friday.
The Colombian stalked out of a pre-Australian Grand Prix news conference after it was hijacked by a satirical television sports show.
Chief operations engineer Sam Michael said Williams, runners-up to Ferrari last season, were "already miles in front of where we were last year".
The Colombian has promised Williams he will do all he can to help them win the constructors' title.
Former Formula One driver Juan Pablo Montoya took a major step in his transition to NASCAR racing when he qualified for the Ford 400 Nextel Cup race at Miami Homestead Speedway.
David Coulthard grabbed second place for McLaren, ahead of Jarno Trulli and Fernando Alonso in the two Renaults.
The former McLaren Formula One driver's car was tapped from behind on lap 252 of a 267-lap race at the Miami Speedway.
The FIA confirmed it had written to teams using Michelin tyres outlining new rules for measuring tread width at the September 14 Italian Grand Prix.
India's Narain Karthikeyan finished last, at 20, with a classification time of 1:51.675
Rubens Barrichello was 0.128 slower than Montoya in second place, while Schumacher was third with 1:20.784.
