MILWAUKEE -- Brewers reliever Tyler Thornburg's abrupt exit in the eighth inning on Thursday was the result of nothing more than a tired arm, the rookie right-hander said after Milwaukee's 8-2 loss to the Nationals.

Facing a 3-2 count against Roger Bernadina to lead off the eighth inning, Thornburg was visited on the mound by a trainer and manager Ron Roenicke. He then left the game, allowing for Kameron Loe to fan Bernadina one pitch later.

Pitching on consecutive days for the first time this season, Thornburg started the seventh inning for Milwaukee, giving up one run on a walk and two singles. On the first pitch of the eighth inning, Roenicke said catcher Martin Maldonado noticed something was wrong.

"[He] looked OK, delivery looked fine, but the velocity was down, so I went out there," Roenicke said. "He said he's not sore at all, just probably getting used to the bullpen and coming out back-to-back days."

Thornburg, who is ranked by MLB.com as Milwaukee's No. 4 prospect, threw 15 pitches and gave up two hits in one scoreless inning on Wednesday before returning to the hill on Thursday. In his first season in the Majors, Thornburg has appeared in five games for the Brewers, including two starts. He also made 14 starts in the Minors this season.

Before Wednesday, Thornburg's last appearance was on July 18, when he threw 103 pitches and gave up two earned runs over 4 2/3 innings. To stay sharp with the long layoff, he threw 25-30 pitches off the mound in a simulated session on Tuesday, which might have contributed to his tired arm.

As he continues to adjust to the bullpen, Thornburg said it has been an easy transition when he wasn't throwing often.

"It was always elongated rest," Thornburg said. "It wasn't anything like throw, day off, throw, day off, throw -- nothing of that sort. Unfortunately, I've had a about seven days in between every outing, so I think that made it a little bit tougher."

Roenicke said nothing appears to be wrong with Thornburg physically, but he's not sure what the status of the 23-year-old is moving forward.

"We'll see how he is tomorrow," Roenicke said. "The doctors think he's fine, so we'll see how he's doing."