Nationals starter Dan Haren has been a shadow of his former self this season and continued his stuggles Tuesday night in Colorado, where he surrendered five earned runs and a pair of home runs in a disastrous fifth inning. (Chris Schneider/Associated Press)

In his 11-year major league career, Dan Haren has never struggled like this. After he fired the pitch that was smashed for the second home run he allowed at Coors Field on Tuesday night, Haren turned to watch the flight of the ball. He shook his head in disbelief. He looked up at the scoreboard. He took his place behind the rubber, waiting for the next ball from catcher Kurt Suzuki and hoping to move on.

But after he allowed five runs on eight hits over five disparate innings in an 8-3 loss to the Colorado Rockies, Haren was no closer to the answer. His season-long bout with inconsistency continued. He fought off the hot and dry conditions in the thin air of the Rocky Mountains for four innings before imploding in the fateful fifth inning, another start lost.

“I was dumbfounded,” Manager Davey Johnson said. “He had a low pitch count. I was thinking, ‘Boy he’s going to go seven, eight innings the way he is throwing.’ Then it seemed he started leaving the ball up and not hitting his spots.”

In hitter-friendly Coors Field, the Nationals’ meager offense produced two early runs against right-hander Jhoulys Chacin and then slept for the next five innings. Haren allowed a game-tying two run homer to Tyler Colvin in the fifth inning and five batters later coughed up a critical three-run, opposite-field shot to slugger Carlos Gonzalez. No other pitcher in the major leagues has allowed as many home runs as Haren’s 17. Behind his struggles, the Nationals dropped under .500 for the fifth time.

Haren has proclaimed he is healthy, and his improved velocity from his injured 2012 season is prime evidence. He has wracked his mind trying to find reasons why he can pitch well for stretches and then collapse the next. His season is confounding.

The Post Sports Live crew discusses the possibilites of why Bryce Harper had a brace on his knee after seeing Dr. James Andrews on Monday. (Post Sports Live)

“I’m getting strikeouts when I need it, for the most part,” Haren said. “Not quite like I used to. Command-wise, I’m not walking guys. I just can’t keep the ball in the ballpark. That’s what it comes down to. Good hitters, too, they’re going to make me pay for my mistakes. I’ve never had so much trouble with homers in my career. I gotta really focus on keeping the ball down. I’m trying to do the best I can. But nothing is really working for me right now.”

At 32 years old and after 1,9491 / 3 innings on his arm, he still appears to possess the stuff needed to pitch in the majors. A sharp observer of baseball, Haren has learned to pitch with less velocity and thrive. Entering Tuesday’s game, his 5.89 strikeouts-to-walks ratio was fourth best in the majors — a sign that his control and stuff are still successful.

“He’s still very capable,” Johnson said. “We’ve got a long way to go.”

But he is coughing up too many hits (11 per nine innings) and too many of those result in home runs (two per nine innings). So far this season, 63 percent of the 46 earned runs he has allowed have come on home runs. When Haren gets hit, the damage has been big.

Haren is averaging only 5.5 innings per outing, the lowest of his career. He also has been hurt by the lowest run support of any starter on the staff, nearly 2.7 runs per game. The Nationals are 4-9 in games Haren starts, the worst winning percentage of any of the team’s starters.

After allowing three home runs and five runs in his last start, one in which Haren admitted that he felt he was letting everyone down, Haren appeared poised to find some welcome order. Earlier in the day, the disappointing Nationals received encouraging news on various fronts. Their depleted roster could soon have some of its key starters back. Bryce Harper’s left knee didn’t undergo surgery and had no structural damage, and instead received injections. Ross Detwiler could rejoin the starting rotation on Thursday and Stephen Strasburg is slated to follow on Sunday.

Haren could provide more. He did well to start in the unfriendly conditions for a pitcher. The temperature at first pitch was a blazing 95 degrees, a dry heat that makes it harder for pitchers to grip the ball because there’s not even a drop of moisture. That, coupled with the thin air and band box of a stadium, would seem to have spelled trouble for Haren.

Through the first four innings, Haren kept his pitches on the edges of the plate and low in the strike zone. He struck out five, at one point retired 10 straight and allowed only two hits. Then, in the fifth inning, his pitches began drifting up or catching too much of the heart of the plate.

A fifth-inning leadoff walk to Wilin Rosario was followed by Colvin’s homer. He then yielded a single, double and Gonzalez’s blast.

Relievers Erik Davis and Fernando Abad held down the Rockies in the following two innings, but Drew Storen surrendered three runs, including another home run to Colvin, in the eighth inning.

At the plate, the Nationals’ offense did little and the struggles have stretched into the third month of their season. Washington ranked near the bottom in the majors in several major offensive categories entering Tuesday’s games: 27th with a .234 average, 29th with a .291 on-base percentage and 26th with a .375 slugging percentage.

Last year’s Nationals, too, were horribly sluggish offensively to start the season. What sparked the improvement, however, was a trip last June to Colorado. The Nationals posted double-digit run totals in three of the four games. Before Tuesday’s game, Johnson joked that he hoped this trip to hitter-friendly Coors Field would produce the same results. It happened in only one inning.

Against Chacin in the second inning, Ian Desmond led off with a walk. Anthony Rendon singled to center. With one out, Haren’s bunt put both Desmond and Rendon in scoring position. Denard Span roped a change-up to right field to plate both runs. The Nationals would score only one more run after that.

The Nationals would retreat to a quiet clubhouse following another loss, none more puzzled than the man who started the game on the mound for them.