Anything can happen during a match and sometimes some of the referee's decisions will not be in your favor. The fans complained, the coaches complained, and so did the gamblers who felt they deserved to win their bets. Things like that happen often and are quite disappointing, that's what Ten Hag said after the match even though he felt his team had done their best.
Yes, this will clearly not be beneficial for Manchester United who feel at a disadvantage with VAR because they will claim it was not offside but the referee and VAR assistant have given a decision with analysis that it is offside, especially if the goal occurs it will be Manchester United's win, but the facts different. 
This often happens in football matches, not only is this the only controversial incident, but several others have also experienced the same thing, just accept the referee's decision and that is the final decision.
Obviously Manchester United bettors will be disappointed.
In my opinion, to be honest I think the Var robbed Manchester united of the game and that call wrong because it was clearly not an offside and if the goal was given the game would had been in favour of united cause the would had blocked their defence line instead of coming out to score an equalizer anyways the decision is being made an there's nothing anyone can do about it, Manchester United are left with no choice but to accept defeat and work on performing better on their next match which I feel would be very tough cause they're facing Brighton.