I think the decision to play Steffen was due to several factors, and the back-up keeper issue is a tricky one to resolve.
I do think Guardiola wants to maintain a happy squad, and taking a step back for the minute, he probably does have to promise playing in a number of games each season (10 or more) in order to attract and keep a good backup keeper at the club. If he promised Steffen the cup games, then there he probably needs to play him, but I’m not sure this sort of arrangement really works for the team as once you get to the semis, you need to put your best team out.
I do however think that Guardiola fails to see the FA cup as a priority, and his team selections for the semi-finals over the past few years also back this up. Personally I’d rather he play a really weakened team, with lots of academy players, and risk going out in rounds 4/5 than playing four games in the competition - increasing the players’ workload - only to put a weak team out in the semi. All seems a bit pointless to me, and we also lose momentum at the sharp end of the season. Not to mention asking fans to buy expensive tickets and travel all the way to Wembley for a shit day out.
The issue on Saturday was that in a weakened team we were always going to be pressed high - we seemed to be encouraging it - and this pressure exposed Steffan’s shortcomings with his footwork, whereas Eddie would have just dealt with it. It may just be better to play the No. 2 keeper against shit teams at home in the league than in important cup games against our main rivals.