* Chilton dropped for Belgian GP
* American Rossi to make F1 debut for Marussia
* Chilton is second driver to be replaced for Spa (Adds Chilton statement, background)
By Alan Baldwin
SPA-FRANCORCHAMPS, Belgium, Aug 21 (Reuters) - American Alexander Rossi will make his Formula One debut for Marussia at this weekend's Belgian Grand Prix after the hard-up team sidelined Britain's Max Chilton for unspecified contractual reasons.
Rossi, 22, will race alongside Frenchman Jules Bianchi and become the first U.S. driver to start a grand prix since Scott Speed's last race with Toro Rosso in 2006.
The Californian joined British-based Marussia, who are ninth in the 11-team championship, as reserve driver last month after carrying out the role at tail-enders Caterham.
"It's a very big moment for me and there's a lot to prepare in a short space of time, but on the other hand I have felt ready for this for quite a while now," he said.
"It is also exciting to be given this opportunity at such a fantastic and historical circuit as Spa-Francorchamps. I can't wait to drive the MR03 from tomorrow and I hope to reward the team with a solid race weekend."
Marussia indicated the decision to switch drivers had not been a sudden one, even if it took the paddock by surprise as the sport regrouped after a summer break.
"Naturally we hope to resume normal service with respect to our established race driver line-up as soon as possible, but for now we wish Alexander well for the weekend ahead and we look forward to seeing him in action," said principal John Booth.
A statement issued on behalf of Chilton, who was at the Spa circuit on Thursday but did not carry out his scheduled media activities, suggested the 23-year-old Briton had stepped aside of his own volition.
"Max Chilton has volunteered to step out of his race seat for this weekend's race in Spa...to allow the team to attract much-needed funds by selling his seat," it said.
"Marussia are currently in talks with several new investors and it is expected the situation will be resolved before the next race."
Chilton, who has been largely eclipsed by Bianchi, has had to bring money to Marussia as part of his deal and his replacement triggered speculation about those payments.
The Briton is the second driver to be replaced abruptly for the 12th race of the season.
Caterham, who are also fighting for survival, dropped experienced but poorly funded Japanese Kamui Kobayashi on Wednesday and replaced him with 32-year-old German Andre Lotterer.
Lotterer is a triple Le Mans 24 Hours winner but will be making his F1 race debut on Sunday. (Reporting by Alan Baldwin, editing by Robert Woodward)