It appears as if Carlos drew a red for Grenoble (Hugo Cianci). That's our guy. Rennes still up 4-0 with a man advantage.
I don't know if this is the reason Boca played, but the usual right back, Rod Fanni, was playing centerback, and the Boca-replacing left back, Danze', played on the right.
Goals from the match: http://watchsocceronline.blogspot.com/2010/01/bocanegra-does-another-90-in-rennes-win.html
Boca getting a solid 90 in a winning effort is great news. Even if it's not every week, getting on the pitch enough to stay sharp is critical for him right now.