The new manager effect will impact a team's performance. But that effect is strongest immediately after an appointment.
Making a change at a key moment in season can have a significant impact on results, but it will be less pronounced earlier in the season as it will have less impact on long term results.
Teams that lose popular managers are worth betting against unless they can find a replacement of a similar calibre.
think it was the right time for the bottom clubs to change managers , however if vieira was popular then this could go drastically wrong for palace.