If Manchester United failed to do better in the next season I am afraid they're going down to the relegation zone, and that will be a made time for them but am certain the club officials will do all that is possible to bring man United back to it past performance different from what we saw this season.
Although Man United performance isn't as people's expected, they have no reason to drop to the relegation zone. Man United is still a big team, they have many skilled and quality players. They only have a problem with the mentality (motivation) and the strategy in playing, so they need a special coach to encourage the players' performance. Sure, it may be impossible to bring back the performance as in the Ferguson era. But they need to improve their current performance to be more competitive in EPL competition. If they cannot improve it, they will never win EPL title for sure.
Ferguson is not the last good coach for Man U... It's just that the club management hasn't been able to find a really good coach yet... Man U have really good players and superstars, but without quality club management they will never get good results... The situation reminds me of Barcelona when they played horribly, but after Xavi came in, they were able to finish second in the league... About the same decision is needed for Man U...