Saracens are the first club to stage a European match in Brussels - with Farrell coming in as one of four changes.
Winger Chris Wyles, who will make his 100th appearance for Saracens, and scrum-half Neil de Kock also come into the pack.
Up front, there is just the one alteration following the 45-0 rout of Edinburgh - with Rhys Gill preferred at loosehead prop.
Mako Vunipola, Joel Tomkins, Richard Wigglesworth and David Strettle drop to the bench, where they are joined by Alistair Hargreaves and Carlos Nieto.
Racing have recalled Juan Martin Hernandez after the Argentina star missed last week's victory over Munster.
Hernandez starts at full-back, with Benjamin Fall moving to the wing alongside Juan Imhoff.
There is another change in the pack where Bernard le Roux is preferred over Masinivanua Matadigo.
Saracens: 15 Alex Goode, 14 Chris Ashton, 13 Owen Farrell, 12 Brad Barritt, 11 Chris Wyles, 10 Charlie Hodgson, 9 Neil de Kock, 8 Jackson Wray, 7 Will Fraser, 6 Kelly Brown, 5 Mouritz Botha, 4 Steve Borthwick, 3 Matt Stevens, 2 Schalk Brits, 1 Rhys Gill.
Replacements: 16 John Smit, 17 Mako Vunipola, 18 Carlos Nieto, 19 Alistair Hargreaves, 20 Andy Saull, 21 Richard Wigglesworth, 22 Joel Tomkins, 23 David Strettle.
Racing Metro: 15 Juan Martin Hernandez, 14 Benjamin Fall, 13 Mirco Bergamasco, 12 Fabrice Estebanez, 11 Juan Imhoff, 10 Olly Barkley, 9 Maxime Machenaud, 8 Bernard le Roux, 7 Jacques Cronje (capt), 6 Antoine Battut, 5 Francois Van Der Merwe, 4 Karim Ghezal, 3 Luc Ducalcon, 2 Dimitri Szarzewski, 1 Andrea Lo Cicero.
Replacements: 16 Benkjamin Noirot, 17 Eddy Ben Arous, 18 Juan Pablo Orlandi, 19 Fabrice Metz, 20 Julien Jané, 21 Masinivanua Matadigo, 22 Sebastien Descons, 23 Gaetan Germain.
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