London, April 6 -- Arsenal manager Mikel Arteta acknowledged his team is going through its first tough period of the season after being stunned by second-tier Southampton in the FA Cup quarter-finals, two weeks after losing in the final of the English League Cup. A shocking 2-1 loss at St Mary's Stadium left Arsenal with just the Premier League and Champions League as trophy targets. A knee injury to Gabriel Magalhaes, which forced off the Brazil center back in the second half, added further misery for Arteta, who suggested that there were players who took the field against Southampton despite having fitness problems of their own. What Arteta wasn't prepared to do was criticize his squad. "I'm going to defend them more than ever. If someone has to take responsibility that's me and we have the most beautiful period of the season ahead of us," he said. "And now it's a moment in the season - you always have moments, usually two or three. This is the first moment that we have with a certain level of difficulty. "When I say difficulty, we're going to play in the Champions League quarterfinals and the run-in for the league," Arteta added. "It is a difficult period but I believe that many other ones are much more difficult." Barcelona's Robert Lewandowski came off the bench to score a lucky late winner in a 2-1 victory away to 10-man Atletico Madrid that moved the holders seven points clear of Real Madrid at the top of the LaLiga table. The hosts took the lead six minutes before the break through Giuliano Simeone. The visitors responded with a Marcus Rashford goal three minutes later and Atletico had Nico Gonzalez sent off on the stroke of halftime. Lewandowski was in the right place to net a fortunate rebound three minutes from time. The sides will meet again at the Camp Nou on Wednesday in the first leg of their Champions League quarter-final clash....