Foster was a right-handed pitcher with the Boston Red Sox from 1913 to 1917 and won two World Series championships with the team in 1915 and again in 1916.

Foster was picked up by the Boston Red Sox and made his major league debut for the team on April 10, 1913.

Foster was only behind his Boston Red Sox teammate, Dutch Leonard, who posted a 0.96 ERA, which is now considered the modern day all-time single-season record.

Foster most effectively showed his importance to the team in the 1915 World Series where he picked up 2 complete game wins and only gave up 4 earned runs and struck out 13 batters in 18.0 innings.

As a hitter, Foster posted a .215 batting average (58-for-270) with 1 home run, 19 RBI and 16 bases on balls.