MANCHESTER, England, Jan 22 (Reuters) - Manchester United"s Marcus Rashford scored a last-second winner as they beat West Ham United 1-0 at Old Trafford in the Premier League on Saturday to move up to fourth spot in the standings. Substitute Rashford tapped in a low cross from Edinson Cavani deep into stoppage time after United had broken down the left flank with the final move of the match. The win means United move above West Ham with 38 points from 22 matches while the Hammers are now fifth on 37 from 23. Until Rashford"s goal finished off an attack started by Cristiano Ronaldo and also featuring Anthony Martial there had been few chances at either end in a disappointingly dull game. David Moyes" West Ham had been cautious in their approach although Tomas Soucek had gone close to scoring for the Londoners near the end with a glancing header from a corner which flashed just wide of the far post. Fred had United"s best clear opening, shortly after the interval, but his shot was too close to West Ham keeper Alphonse Areola who easily saved. United coach Ralf Rangnick started with Ronaldo at centre forward with 19-year-old Anthony Elanga and Mason Greenwood to his left and right but the combination produced very little. Rashford was brought on for Elanga in the 62nd minute and then 20 minutes later Rangnick rolled the dice by throwing on Cavani and Martial and switching to a 4-2-4 formation. The move paid off when all four combined for the winner in the third minute of the three minutes added on. Ronaldo fed the ball into Martial who slipped it outside to Cavani. The Uruguayan looked close to offside before he hit a cross to Rashford, who scored from the point blank range. VAR looked back at the goal and there was a second roar around Old Trafford as it ruled Cavani had been onside. "The atmosphere (in the dressing room) is amazing," said Rangnick. "Quite rightly the boys were celebrating. They know what a massive win this was. "Those are the best kind of wins when the other team has not time to come back. We are extremely happy with the performance and the level of physicality we played with bearing in mind we played three days ago at Brentford," he said. Moyes said his side had made mistakes in the final moments and paid a high price for them. "It"s not easy when you lose a goal in the last second. It was certainly avoidable. We made a couple of poor decisions in the build-up to it," he said, before defending his conservative approach to the game. "I want to win. I’m not coming to these teams to let them beat me. I’m coming to challenge them. I can’t fault the players for their effort. "But we just lacked little bits and did not do the correct things, how to see games out and we didn’t do that in the last minute."