Fontenot was still in Cubs gear a couple hours before game time, ready to make the short walk across the ballpark — all of about 500 feet. He was available for the Giants off the bench.
"It's been a strange day so far," Fontenot said. "Coming to the field to get your work in and went up the stairs after hitting and they called me in the office. I'm excited for the opportunity. Knowing the Giants are in the thick of it really gets you pumped up."
San Francisco sent speedy Class-A center fielder Evan Crawford to the Cubs, who weren't sure yet at which level Crawford would play.
The Giants were in negotiations with the Cubs to acquire Fontenot at the July 31 trade deadline. Talks picked up between Cubs general manager Jim Hendry and Giants GM Brian Sabean after San Francisco shortstop Edgar Renteria strained his left biceps and went on the disabled list Wednesday, Renteria's third stint on the DL this season.
Renteria underwent an MRI exam earlier Wednesday for the injury that began bothering him on a swing during last week's series at Atlanta.
"Mike will help us in the infield, give us experience and a left-handed bat," Giants manager Bruce Bochy said. "I wish we had him a day earlier."
The 30-year-old Fontenot is batting .284 with one home run, 11 doubles, three triples and 20 RBIs in 75 games this season. Of his 40 starts, 29 have been at second base, seven at third and four at shortstop.
He could spell Freddy Sanchez at second while also filling in at other positions.
The Giants also activated infielder Eugenio Velez from the disabled list and optioned him to Triple-A Fresno. Velez was struck on the head July 25 by a foul ball while in the dugout against Arizona.