In the Washington Redskins’ first few games of last season, when No. 13 in the burgundy and gold took his spot at wide receiver, few opponents knew who he was, much less what to expect. Anthony Armstrong went undrafted out of West Texas A&M. He spent the 2008 and 2009 seasons on the Miami Dolphins’ practice squad and had never appeared in a regular season game. Why account for him?

Armstrong, though, made teams notice, and even alter their coverage. In a season of uneven offensive play for the Redskins, he became a potential game-changer. By season’s end, he averaged 19.8 yards per catch — third-highest in the league — and had seven catches of 45 yards or more.

Yet entering Sunday’s game against San Francisco, Armstrong is just another struggling element of a struggling offense. He suffered a hamstring injury in Week 3 at Dallas. He sat out the next two games. He has all of five catches for the season. The longest of those is 18 yards, nearly two yards below his average last season.

“It’s kind of frustrating,” Armstrong said this week, before quickly recalibrating. “It’s really frustrating that you aren’t able to really get on top of defenses and take advantage that way.”

Armstrong’s problems, though, are indicative of an offense that, as the midpoint of the season approaches, is solidifying its standing as one of the NFL’s worst. Through seven games, Washington has shown almost no big-play capability. The Redskins have just two pass plays of at least 40 yards, the longest a 45-yard toss from Rex Grossman to wide receiver Jabar Gaffney in a Week 6 loss to Philadelphia. Just two teams have failed to complete at least one pass longer than that.

Go deeper. Redskins wide receivers have just five receptions of 25 yards or longer — three by Gaffney and one each by Terrence Austin and Santana Moss, currently out because of a broken hand. Only one team, Minnesota, has fewer long completions to wideouts. The trickle-down is felt throughout an offense that this week faces a San Francisco defense that is the best in the NFL against the run.

“It’s tough to make long drives down the field when you move five yards at a time,” offensive coordinator Kyle Shanahan said. “You’ve got to get some big plays in there to get points. We weren’t able to get those last week, and I think that’s one of the main reasons we struggled.”

In that 23-0 loss to Buffalo, quarterback John Beck took several shots downfield and completed none of them. He threw four balls in Armstrong’s direction. Armstrong finished without a catch. That feeling he had those first few weeks of 2010, when he surprised almost everyone? It’s gone.

“I think people kind of know what I’ve been able to do for our team last year,” Armstrong said, “and they’re probably just saying, ‘All right, when he’s out there, let’s back up a little bit and make sure he doesn’t beat us that way.’ ”

Redskins coaches believe that’s just part of the equation.

“Anytime you’re banged up and you have a pulled hamstring and all of a sudden your speed is not there, and you’ve got to get back into football shape, and [the opponent is] much more aware of his speed and [he’s facing] a little bit more bump coverage — it’s a combination of those things,” Coach Mike Shanahan said. “He’s feeling better right now, so he’s more in football shape so he’s got an opportunity now, more so than over the last few weeks.”

One thing Armstrong doesn’t have is the comfort level Moss provided by catching balls underneath. Last week against Buffalo, Gaffney and Armstrong were the starting wide receivers, with Donte Stallworth, Niles Paul and Austin also active. Moss’s absence, which might last another month, can affect them all.

“When people would focus on covering Santana with two people, that would leave the other receiver one-on-one, and you were able to take advantage of that other DB,” Armstrong said. “Now defenses have a little bit of a different plan when Santana’s out. We just got to find a way to take advantage of that.”

Kyle Shanahan said some of the deep balls were open against Buffalo. “The ones we’ve had,” he said, “we’ve missed.”

Perhaps this week that will change. The 49ers’ defense is a huge reason they’re 6-1, but just four teams have allowed more pass plays of 25 yards or more than San Francisco’s 17.

“Hopefully, eventually something will break and everything will line up,” Armstrong said. “All the stars will align, and we’ll be able to get a deep pass on somebody.”