Now a second one seems quite probable.
De La Rosa tossed seven innings and gave up one run and five hits to help Boston (45-52) win for the sixth time in seven games as it continues to try to climb out of last place in the AL East.
Duffy also had a strong outing for Kansas City (48-48), allowing two runs - one earned - and six hits in 6 2-3 innings. The Royals lost their eighth in 11 games and fell to 10-20 in one-run contests after leading the AL with 31 such victories last season.
"We're not scoring enough runs. It's as simple as that," manager Ned Yost said. "When we score, we win."
Now the Red Sox turn to Lester (9-7, 2.65 ERA) as he tries to extend his unbeaten streak to seven starts. The left-hander is 3-0 during his run with a 1.01 ERA, lasting at least seven innings in each while Boston has won all six contests.
He went seven and gave up one run with 12 strikeouts against the Chicago White Sox on July 10, but was denied a victory when Koji Uehara allowed two runs in the ninth. Boston went on to win 4-3 in 10.
"I was dropping (my curveball) in for strikes and bouncing it, too," Lester told MLB's official website. "When I'm able to do that, I can get some separation from my fastball and cutter. It widens the plate for me."
Lester has been dominant in 10 career starts against Kansas City - particularly at Fenway, where he's gone 5-0 with a 1.02 ERA in six outings and threw a no-hitter on May 19, 2008.
Though the results have been inconsistent, it's been a solid season overall for hard-throwing Royals rookie Ventura (7-7, 3.22), who is again slated to start after making his first career relief appearance just before the All-Star break.
Ventura, who has never faced Boston, has allowed more than three runs in only four of his 17 starts, but he's received two or fewer runs of support in five of his losses.
His most recent appearance was on July 13, when he came on in relief of Bruce Chen and earned the win with 1 2-3 scoreless innings in a 5-2 victory over Detroit.
Ventura will be looking to bounce back after his previous start was his shortest of the season and ended with him failing to get a decision in a 5-4 win at Tampa Bay on July 9. He tossed five innings and allowed five runs with five strikeouts.
Red Sox outfielder Shane Victorino, who had been on the disabled list since May 24 with a strained right hamstring, was activated before Saturday's game and went 1 for 3.
The Royals started Jarrod Dyson in center in place of Lorenzo Cain, who is hitless in his last 19 at-bats. Yost said Cain would be back in the lineup Sunday against Lester.