The Bees are still having to cope without Yoane Wissa (ankle), Rico Henry, Josh Dasilva, Igor Thiago (all knee), Aaron Hickey (thigh) and Gustavo Nunes (back) due to injury.
Mikkel Damsgaard (knock), Christian Norgaard (muscle) and Mathias Jensen (calf) were all unable to represent Denmark during this month's international break and the midfield trio are doubts ahead of this weekend's contest.
Thomas Frank deployed a four-man defence in Brentford's 5-3 victory at home to Wolverhampton Wanderers prior to the international break, but he may consider reverting to a three-at-the-back system against Man United, with Sepp van den Berg, Ethan Pinnock and Nathan Collins all starting in central defence.
Kristoffer Ajer - who scored a dramatic 99th-minute equaliser in a 1-1 draw with Man United in March - could move over to right wing-back, while Keane Lewis-Potter operates on the left, with Vitaly Janelt and potentially two of Damsgaard, Norgaard or Jensen - if fit - all linking arms in centre-midfield.
Bryan Mbeumo is Brentford's top scorer so far this term with six goals in seven Premier League games and he is set to continue in attack alongside either Kevin Schade or Fabio Carvalho.
Brentford possible starting lineup: Flekken; Van den Berg, Collins, Pinnock; Ajer, Jensen, Norgaard, Janelt, Lewis-Potter; Schade, Mbeumo
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