Dustin Pedroia, Adrian Gonzalez and Kevin Youkilis each had three hits and drove in a run for the Red Sox, who have won three straight.
Boston has scored 17 runs in the last two games after managing just 15 in the previous six games.
Navarro hit a solo shot in the seventh for his first major league homer. McDonald’s drive in the eighth landed on the train tracks atop the wall in left field and stretched the lead to 9-3.
One night after rallying for six runs in the seventh inning of a 7-5 victory over the Astros, the Red Sox got off to a fast start against J.A. Happ.
Adrian Gonzalez had an RBI single and David Ortiz, playing first base in the NL park, walked with the bases loaded to force in another run as Boston jumped out to a 3-0 lead in the first inning. Pedroia added a run-scoring double in the fifth.
That was more than enough for Andrew Miller (2-0), who yielded two runs and seven hits in six innings.
Happ (3-10) left with two outs in the seventh and was charged with five runs and seven hits. He has dropped a career-worst six consecutive decisions and Houston has lost his last nine starts.
Hunter Pence homered and drove in two runs for the last-place Astros, who have dropped 13 of 17. Michael Bourn tied a career high with four hits.
Navarro, who was called up from Triple-A Pawtucket before Thursday’s game, hit his homer to the first row of the Crawford Boxes in left field with one out in the seventh. It was the second pinch-hit homer by a Red Sox player this season.
Boston reliever Alfredo Aceves gave up consecutive singles to pinch-hitter Jeff Keppinger and Michael Bourn with two outs in the bottom half before walking pinch-hitter Matt Downs. He was replaced by Daniel Bard, who walked in a run before retiring Carlos Lee to limit the damage.
Boston started the eighth inning with three consecutive singles and Jarrod Saltalamacchia‘s sacrifice fly made it 6-3.
Bourn extended his hitting streak to 11 games with a leadoff triple in the first. Angel Sanchez drove him in with a single.
Ortiz, normally the designated hitter, played first and Gonzalez moved to right field to start Saturday’s game. Gonzalez moved to first in the bottom of the sixth when Josh Reddick replaced Ortiz.
Miller was struck on the back by a ball hit by J.R. Towles in the fifth inning. Trainers talked with him for a couple of minutes before he threw a warmup pitch and remained in the game.
Notes:
Red Sox OF Jacoby Ellsbury returned after sitting out Friday’s game due to illness. “I don’t think he feels great,” manager Terry Francona said, “but he’s good enough to play.” … Boston OF Carl Crawford jogged in the outfield and took batting practice, as he continues to recover from a strained hamstring. Crawford is due to come off the disabled list on Sunday, but Francona said Crawford isn’t ready to play just yet. “He’s building,” Francona said, “and it was a really productive day.”