He is important but Messi is much more important. We did, after all, beat two top European teams without Busi for a large portion of both games.
One of those "Top European teams" has only made it out of the group stage in the CL twice, losing to us both times. Beating Man City is about as impressive as beating Ajax or PSV as far as European competition goes.
And when playing against Madrid it was clear from kickoff that we desperately needed Busquets and it could even be argued that Madrid's counter wouldn't have led to a goal if we had him there to recycle possession for us.