The radar gun revealed no signs of trouble. But Dan Warthen knew better.

The Mets' pitching coach watched Matt Harvey warm up before his last start. He didn't like what he saw. He could almost predict what came next.

"You can see it in their face," he said later as he discussed the common pitchers' fatigue known as "dead arm."

Sure enough, Harvey got knocked around by the Pirates on Saturday, sent to the showers after the worst start of his big-league career.

He did it while throwing 96-mph fastballs. That might have been plenty had there been any life in his arm. But with seven runs and six hits in four innings, the Pirates quite loudly told him that there was none left.

"There still wasn't that finish that Matt has," Warthen said. "There's 96 and there's 96."

Harvey will return to the mound Friday night for the first time since that episode, taking the ball in the opener of a three-game series against the Marlins at Citi Field. All eyes will be on him, anxiously awaiting the first indication that he's past his first speed bump in an otherwise smooth return from Tommy John surgery.

The Mets have acted and sounded as if they're content with the health of their 26-year-old rock.

With righthander Dillon Gee ready to come off the disabled list, the Mets easily could have skipped Harvey in his turn through the rotation. They also could have given him an extra day's rest and activated Gee to start Friday night.

Instead, the Mets extended Gee's DL stay, partly so Harvey wouldn't have to deal with a routine-wrecking second day of extra rest.

Warthen said Harvey (5-2, 2.91 ERA) looked good during his throwing sessions this week. Manager Terry Collins brushed aside any thought of lingering issues. "There's no concern," he said.

Harvey consistently has insisted that his arm feels fine, and he didn't deviate from the script after his start against the Pirates.

Still, Warthen said pitchers generally endure dead arm once or perhaps twice during the season. It tends to crop up with all pitchers, regardless of whether they're coming off time missed to surgery, as Harvey is, or if they're pitching in a six-man rotation, which the Mets are about to adopt.

"Jon Niese went through it a couple of times ago. Harv's gone through it, [Jacob] deGrom's gone through it," Warthen said. "It is one of those things that happens."

Through the years, Warthen has heard pitchers describe the sensation in various ways.

"One's 'my shoulder feels like it's a hundred pounds' or 'I have a toothache in my biceps,' " he said. "There's different explanations."

For power pitchers, a live arm allows for more explosiveness. Fastballs, Warthen said, arrive at the plate "like it gets a second gear."

Part of the deception comes from the ease of the delivery.

"It's so easy, it's so compact, the foot lands and the arm gets through so quickly," he said.

During a dead-arm phase, however, that ease is replaced with strain. That shift alone can be enough to throw off a power pitcher's mechanics.

"When he tries to throw 96, he jumps out [of his delivery], he opens up and it looks like he's throwing from second base," Warthen said.

With a better view of the ball all the way through the delivery, hitters can tee off, even if the velocity on the radar gun shows nothing different. But in Harvey's throwing sessions this week, Warthen liked what he saw. Harvey's mechanics looked smooth again.

There were signs of life.

On Friday night, the Mets will learn if their ace has pushed through. The ace's pitching coach is optimistic. Said Warthen, "I've seen that he's come out of the dead arm."