Gabriel was forced off with a hamstring injury after just 36 minutes during Arsenal 's 2-1 victory at QPR on Wednesday night , and Arsene Wenger admitted the defender could be out for three weeks . The £16m January signing was making his second Premier League start for the Gunners having made his full debut against Everton last weekend , but he went down holding the back of his leg in the first half . Laurent Koscielny replaced the 24-year-old Brazilian , who is set to miss the crucial FA Cup quarter-final clash with Manchester United at Old Trafford . Arsenal 's new signing Gabriel was forced off with a hamstring injury in the first half against QPR . Arsene Wenger says he expects the Brazilian defender to be out for around three weeks with the injury . Laurent Koscielny replaced Gabriel with just 36 minutes on the clock at Loftus Road on Wednesday night . 'Gabriel had a hamstring problem , ' Wenger said after the game at Loftus Road . ' I do n't know how bad it is yet , it is usually 21 days . ' The former Villarreal centre back was given a torrid time early on by Bobby Zamora as he got used to the physical nature of the Premier League . Wenger added : 'We were nervous in the first half and QPR put a shift in physically . They paid for that in the second half . ' Arsenal sealed the win with goals from Olivier Giroud and Alexis Sanchez , though in-form striker Charlie Austin gave Wenger 's side a fright late on . Koscielny is expected to partner Per Mertesacker as the Gunners travel to play Louis van Gaal 's Red Devils on Monday night , looking to reach Wembley for the second successive season . The former Villarreal centre back was given a torrid time by Bobby Zamora early on in the 2-1 victory . Per Mertesacker is expected to keep his place and partner Laurent Koscileny against Manchester United .
Arsenal beat Premier League strugglers QPR 2-1 at Gabriel . Loftus Road , a £16m January signing , was forced off after 36 minutes . Arsene Wenger says Brazilian defender could be out for 21 days . Per Mertesacker and Laurent Koscielny set to start at Manchester United . Olivier Giroud and Alexis Sanchez scored goals against QPR . CLICK HERE for all the latest Arsenal news .