The Chicago Tribune is reporting the deal is worth $25 million and will be announced as soon as a spot on the roster is cleared for LaRoche.
The 35-year-old just finished his 11th season in the majors as he batted .259 with 26 home runs and 92 RBI in 140 games for Washington.
In 1,478 games with Atlanta, Pittsburgh, Boston and Washington, he has a career average of .264 and has clubbed 243 home runs with 838 RBI.
The Spurs have won three straight since a road setback to the Sacramento Kings one week ago. They demolished the Philadelphia 76ers by 25, then pulled out a great win against LeBron James and the Cleveland Cavaliers by two in Cleveland on Wednesday.
San Antonio collected its third consecutive victory Friday night with a 121-92 drubbing of the Minnesota Timberwolves at the Target Center.
Tony Parker had 28 points and five assists to lead the Spurs.
Danny Green knocked down four 3-pointers and finished with 18 points, while Kawhi Leonard added 14 for the Spurs, who have won six of their last seven games.
"Our mindset was to be focused from the get-go and make sure we can rest the guys in the second half," Parker said.
Marco Belinelli returned to the Spurs' lineup Friday after missing eight games with a groin injury. He scored five points in 10 minutes.
Tiago Splitter and Patty Mills are still out and Matt Bonner missed his second straight with a stomach virus. Corey Joseph hurt his ankle against Minnesota and didn't return. His availability for Saturday is unknown.
The Nets ended a five-game slide with a 94-92 road win over the Oklahoma City Thunder on Friday. The victory also halted a three-game road losing streak for Brooklyn.
Reggie Jackson's last-second 3-pointer rimmed out, and the Nets survived. Brooklyn trailed by 10 late in the third quarter, but a 12-0 run that extended into the fourth made it a 76-74 game. A 10-0 run later in the fourth gave the Nets a seven-point lead, but Jackson and the Thunder mounted a final rally.
"All those buckets were huge," Nets coach Lionel Hollins said of the 10-0 surge.
Jackson scored six of Oklahoma City's last nine points, with Anthony Morrow supplying the other three on a triple with just under two minutes to play. Jackson's layup with 5.5 seconds left made it 93-92, and Joe Johnson split a pair of free throws at the other end.
Out of a timeout, Jackson isolated at the top of the key and pulled the trigger, only to have his potential game-winner fall out. Kevin Garnett secured the rebound, and the game.
Jarrett Jack scored a game-high 23 off the bench to lead the Nets. Deron Williams and Brook Lopez supplied 17 and 16, respectively. Mirza Teletovic chipped in 13 in a reserve role as the Nets overcame 19 turnovers.
The Nets finish off this road trip Wednesday in Philadelphia.
The Nets beat the Spurs on Feb. 6 to snap a six-game losing streak. The Spurs are 20-2 versus the Nets and won 11 consecutive home games in the series. The last regular-season game the Nets won in San Antonio came on Jan. 22, 2002.