That was by far the lowest of his career. So another reason why that season was so great. I did not see very much on this. See The Great Forgotten Season: Carl Yastrzemski, 1967 by Cody Swartz of "Bleacher Report."
Yastrzemski's only season when he had a higher AVG against lefties was 1967. He also hit especially great after August 31. He batted .417 (40 for 96). He ended August with a .3085 AVG. He slugged .760 after Aug. 31. Through that date it was .594. See Was The Left Hand Of God Responsible For The Red Sox Miracle In 1967?.
That season was also one of the most indispensable seasons ever, meaning his team really needed him to have a great year. See Indispensable Seasons Go To WAR!
The table below show's his GDPs and GDP rate for each year of his career. 1967 was much lower than any other year. I also show his SO rate for each year (using PA - IBB - SH). Data from Baseball Reference. I thought maybe if his strikeout rate had been alot higher that year it would account for the lower DP rate. But it does not look that way. The one thing Yaz did that year that he never did before was hit alot of HRs, 44. His previous high had been 20. So maybe putting the ball in the air more helped. Baseball Reference does have his FB/GB ratios for any year. But in other years when he hit 40+ HRs, the GDP rate was not so low (1969 & 1970). Maybe he was just faster that year.
How many runs did he save by hitting into fewer DPs? Averaging his rate over 1966 and 1968, I get about 9 DPs saved. What was it worth to not hit into a DP? Using Tangotiger's Run Expectancy Matrix, 1950-2010, my guess is that it would be worth between .122 and .198 runs in each case. Using the 1950-68 matrix, with a man on 1st and 2 outs, the run expectancy is .264. With 2 outs and none on, it is .066. So if there is a man on 1st and no outs and Yaz strike's out or beats the throw to first, you save .198 runs. Doing something similar with a man on 1st and 1 out gets us .122 runs saved.
The average of those two is .16 and over 9 DPs avoided, it is just 1.44 runs. That may not be much but when the Red Sox and Twins played the last game of the season tied for first, the lower DP total might have mattered.
If I used run values from the 1999 Big Bad Baseball Annual, a GDP was -.37 runs and other outs were worth -.09. So not hitting into a DP and making another kind of out saves .28 runs for a total of 2.52 runs over 9 DPs avoided.
Now maybe Yaz got hits instead of hitting into DPs. But the two other years when he hit over .320 he had about a 10% GDP rate, still much higher. So we can't simply say he hit better that year and that caused the lower GDP rate.