"I thought it was a little tougher bringing him out for the eighth, but that's where we are," Showalter said.
Darren O'Day replaced Tillman and served up Coco Crisp's leadoff home run in a 2-1 loss to the Athletics.
Tillman held the A's to one run and three hits, with two walks and nine strikeouts. He was gone after 115 pitches, two short of his season high.
"I think he's leading our club now in innings pitched - 163, somewhere like that," Showalter said. "But no, he emptied the tank there in the eighth inning."
Tillman has thrown 160 innings this season. Bud Norris is next with 153 2/3. The next-closest Oriole is Miguel Gonzalez with 134 2/3.
How good was Tillman today?
"As good as (Jarrod) Parker, and that's pretty good," Showalter replied. "Got to see two guys at the top of their game. August-September baseball. That was pretty impressive to watch. Both of them were as advertised. It's a pity neither one of them got a W in the game.
"That was pretty impressive by both of them. You could tell early on that Parker was carrying really good stuff and that runs were going to be at a premium."
Tillman seemed to get better as he went along, retiring the last seven batters he faced with four strikeouts.
"He does that a lot," Showalter said. "Like I said many times, one of the differences with him is he may get behind 2-0, 3-1, 3-0, and next thing you know he comes right back and he's got back his sink. And we're not having to make those constant mound visits.
"I'm real proud of him. He's solid. I think the last pitch he threw was 95 mph, I saw on the board real quick while I was trying to see if he was at 140 pitches yet. When you go back out and pitch the eighth inning like he did, that's impressive. Parker matched him, which they needed.
"Tilly bent, but didn't break a couple times. It's frustrating. We end up on the losing side, but watching two guys at the top of their craft matching each other, you knew there was going to be a fine margin of error."
O'Day actually committed a throwing error in the ninth, only the 39th miscue by the Orioles this season. His biggest mistake was the pitch to Crisp that resulted in the seventh homer off O'Day in 55 1/3 innings.
"Darren's been great for us," Showalter said. "Obviously, on paper, that's not the thing I'll dwell on. Darren came back and got some outs for us."
Left-handers were batting .324 vs. O'Day before today, compared to .161 by right-handers. The splits haven't been that extreme throughout his career. Left-handers were batting .252 against him, compared to .202 by right-handers.
"Something he's pretty good at since he's been here," Showalter said. "He was one of few rested guys down there. Threw the ball well, just really made one mistake. Got behind in the count a little bit there. But you've got to tip your cap to Crisp. He's sitting on a 3-1 breaking ball."