Whenever he encounters struggles or falls short in an assignment, Washington Redskins left tackle Trent Williams takes a simple approach. “Just wipe it out,” he reminds himself and then moves onto the next play. As his downtrodden Redskins put the finishing touches on their game plan for the New York Giants, who will visit FedEx Field on Sunday night, Williams will attempt to wipe it out again. But he is trying to move on from more than just one poor play. In the last month, Williams hasn’t looked like the rock-solid tackle that anchored the Redskins’ line last season, helping his team win the NFC East and earning his first Pro Bowl selection. After a strong start to this season, when he went six straight weeks without allowing a sack, Williams has given up six in the last five games. Then his image took a hit last week when he found himself at the center of controversy following an alleged profanity-laced exchange with a referee. Two weeks ago, following the Redskins’ loss at Philadelphia, Williams seethed in the locker room and revealed that referee Roy Ellison verbally abused him. Williams said Ellison called him a “garbage [expletive], disrespectful [expletive].” Ellison received a one-game suspension from the NFL, but the official reported to the Fritz Pollard Alliance that Williams had used abusive language and directed a racial epithet at Ellison after the official tried to break up a fight between Williams and several Eagles players. Williams, who was not disciplined by the NFL, denied using any derogatory language toward Ellison. The matter seemingly had been laid to rest by the end of last week when Ellison’s suspension was handed down, and Williams said he had moved on. But he couldn’t work his way out of his on-field funk. The Redskins hosted the San Francisco 49ers in what represented a must-win game on “Monday Night Football.” But Williams had the worst game of his season — and arguably one of the worst of his career — as he surrendered two sacks to defensive end Aldon Smith. The No. 4 overall pick of the 2010 draft also surrendered another four quarterback pressures, and Robert Griffin III and the Redskins’ offense failed to score a touchdown for the first time in the last two seasons.