It's easy to see why Reynaldo Lopez is one of the keys to the Chicago White Sox's rebuilding process.

The 24-year-old right-hander, acquired as part of the December 2016 trade with the Washington Nationals for outfielder Adam Eaton, is 4-7 with a respectable 3.91 ERA this season.

He's scheduled to make his 20th start of the campaign Sunday afternoon against the host Seattle Mariners and left-hander Marco Gonzales (10-5, 3.41 ERA).

"He's not afraid,'' White Sox pitching coach Don Cooper told the Chicago Sun-Times regarding Lopez. "He's attacking, he's competing and he's learning. He's (112 2/3) innings into the process of his first full season and is trying like hell to do what he has to do to take the bulk of the games to the finish line.''

Lopez has already taken a leadership role on the staff. Earlier this season, after a poor performance in Cleveland, Lopez said he and his team "played like clowns."

"I thought he wasn't lying," Cooper said of the clown quote. "Listen, your record tells you where you are. Our record as a team isn't what we want it to be. But we're going through a rebuilding year. We knew as an organization that this would be a tough time.''

After a mistake, allowing a home run to All-Star catcher Salvador Perez, cost him a quality start in his last outing against Kansas City, he slammed his pink glove -- a tribute to his daughter born the previous day -- against the dugout wall.

"It was my mistake. I was stubborn to throw that pitch and I paid for it,'' said Lopez, who shook off catcher Kevan Smith's call for a changeup. "I wanted a fastball. Of course, he was ready for a fastball and he hit it."

Cooper said that was the difference between a good game and a "horse (bleep) game."

"Even though he pitched well, that one last mistake turned his line into one that looked like he didn't," Cooper said. "Right now, I'm good with where he's at, I think he's doing fine. But ... we've got about 12, 13 more starts to continue the learning curve.''

That will continue Sunday afternoon in the finale of a three-game series, as Lopez will face the Mariners for the first time. After losing the opener, the White Sox won 5-0 Saturday night as Dylan Covey pitched 8 1/3 innings of two-hit ball and outfielder Avisail Garcia, making his return from the 10-day disabled list because of a right hamstring strain, hit a three-run homer.

Gonzales, 26, has won his past three starts for the Mariners, allowing two runs in 22 innings (0.82 ERA) with two walks and 18 strikeouts. He has made one career start against the White Sox, pitching six scoreless innings April 24 in Chicago in a 1-0 victory.

"He's been huge, no question about it," Mariners manager Scott Servais said of Gonzales. "His work between starts is impeccable. He's really disciplined. A lot of times young players come in and get on a roll and say, 'OK, I've got this figured out.' Marco has not done that. He has not backed off the pedal at all and that's why he's going to have a lot left in the tank as we go into the second half."