Buchholz gave up five runs -- Carlos Correa hit a solo homer and George Springer a grand slam for the Houston Astros -- in the first two innings of Boston's 11-inning win on Saturday.
There were boos as Buchholz walked off the mound in the second. He was charged with five runs in six innings, holding Houston scoreless over his final four.
Buchholz (2-3) has surrendered eight home runs after giving up six in 18 starts a season ago and is sporting the worst strikeout-to-walk ratio (1.60) of his 10-year career, all with the Red Sox. His ERA spiked to 6.11 through eight starts.
"It's not a matter of stuff. I wouldn't say it's pitch selection," Farrell said. "It comes down to location, and that was the case yesterday, whether it's throwing behind in counts, a walk mixed in or a pitch that leaks back to the middle of the plate and is squared up. And it's seemingly showing its head in the first couple of innings."
Buchholz has given up 19 runs in the first two innings of games and just 12 in innings three through seven, according to ESPN.com.
The Red Sox are waiting on the return of left-hander Eduardo Rodriguez, who suffered a knee injury in spring training.
Farrell said Rodriguez will need at least one more rehab start at Triple-A Pawtucket.