"I just didn't make pitches when I needed to," Treinen said on Sunday. "The balls found holes. I didn't put guys away when I was [ahead] in the count. When you make mistakes, hitters will make you pay. That's what happened."
The Nationals were hoping that Treinen could be a setup man, but he had problems with control, walking 21 in 41 innings, and left-handed batters hit .346 against him.
"For Blake, it's about consistency. We know the kind of talent he has," manager Matt Williams said. "The issue that he had this year [dealt] with the left-handed hitters. It's something to work on. Yesterday was an example. He got ahead, but he couldn't finish. We'll give him an opportunity to get down there and get consistent work."
De Los Santos, Washington's 28th-ranked prospect, was part of the deal that sent left-hander Ross Detwiler to the Rangers in December, and he went 4-4 with a 2.65 ERA in 26 appearances for Double-A Harrisburg this season. He has not allowed a run since June 8 -- a 15-inning stretch in which he has held opposing batters to a .104 average and struck out 15.
De Los Santos has struck out 8.4 batters per nine innings this season, and he has posted a 0.884 WHIP.
"He has an above-average fastball. He throws two breaking balls on any given day. They have the ability to get a swing and a miss," said assistant general manager Doug Harris. "From a makeup standpoint, he is an aggressive kid, he has good poise. From that perspective, I think he is capable of handling his surroundings. From a stuff perspective, he has the ability to compete."
Bill Ladson is a reporter for MLB.com and writes an MLBlog, All Nats All the Time. He can also be found on Twitter @WashingNats. This story was not subject to the approval of Major League Baseball or its clubs.