The Red Sox began with another lackluster effort on this miserable road trip before pulling even with a rally that took just six pitches.
Dustin Pedroia and Jacoby Ellsbury opened the sixth with singles off Seattle starter Blake Beavan (9-9) before Ross hit his 20th homer of the season. After recovering to get two outs, Beavan left a pitch up in the zone to Lavarnway and he lined his first homer off the out of town scoreboard beyond the left field bullpens.
The quick turn of fortunes for Boston made a winner out of Jon Lester (9-11), who gave up consecutive homers to Franklin Gutierrez and Kyle Seager in the third, but otherwise avoided damage.
Eric Thames hit a two-out, two-strike triple in the ninth for Seattle off Andrew Bailey to keep the Mariners hopes alive. But Bailey got Seager to fly out to right to end it for his second save in three chances.
Boston's seven-game skid started with the opening of a West Coast swing through Los Angeles, Oakland and Seattle. According to the Red Sox, from information provided by the Elias Sports Bureau, it was the first time Boston had dropped the first seven games of a road trip since 1960. The losing streak was the longest since a nine-game skid during the 2001 season.
The Red Sox appeared ready for the losing to end, with most leaping to the dugout railing to see Ross' homer sail over the head of left fielder Trayvon Robinson. Boston had previously missed scoring chances in the first, second and fifth innings with runners in scoring position.
Pedroia's single to start the inning extended his hitting streak to a season-high 15 games.
Beavan recovered to get James Loney and Jared Saltalamacchia on fly outs before Lavarnway came through with his first home run for the Red Sox since a two-homer game Sept. 27, 2011, against Baltimore.
Lester needed 111 pitches to get through six innings, but worked out of jams in the first and second innings leaving runners stranded at third base. Jesus Montero had three hits off Lester, including a two-out RBI single in the first. Gutierrez homered to deep center field on the first pitch of the third inning and, two pitches later, Seager hit his 16th of the season into the right field seats.
Lester gave up nine hits for the second straight start, but struck out four and won for the fourth time in his last five outings.
Gutierrez's homer was his third of the season, but also ended his night. He left the game before the start of the fourth inning with what the team said was a precautionary move after Gutierrez felt tightness in a groin muscle. Gutierrez made a diving catch in center field in the first inning.
NOTES: Seattle ace Felix Hernandez was named the AL pitcher of the month for August. Hernandez was 4-0 with a 1.08 ERA in five starts, including the 23rd perfect game in baseball history. ... Boston recalled RHP Clay Mortensen from Double-A Portland before Tuesday's game. It was the sixth time this season he's been called up from the minors this season. ... Seattle hit back-to-back homers for the third time this season and first since July 2 versus Baltimore.