Let's do some good news, bad news. Let's start with the bad first.
Bad news: Samir Nasri is done for a while. Pellegrini said that it would be 6-7 weeks in his press conference, so he could be back for the second Champion's League leg. Yaya Toure and David Silva are also out tomorrow, most likely just to get them some rest. Yaya has had some lingering back issues, which hopefully will stay small. It makes sense why he has been getting more games off than Fernandinho has recently. Silva has a knock which will keep him out for the Blackburn match, and possibly against Cardiff on Saturday. Jovetic is still out, and it seems that Pellegrini might be getting a little frustrated with him.
Good news: Sergio Aguero is going to play tomorrow. Our best (or at least co-best with Silva and Yaya) player will be back, most likely coming off the bench to get some match time. Other good news, Micah Richards should be starting. He was on the bench for the Newcastle match, but with how nervy that got, Pellegrini didn't seem to be comfortable throwing him on there. Having Micah back will be a boost to keeping Zabaleta healthy. Pablo is a crazy man, but is pushing the amount of playing time that leads to serious injuries.
Expectations from both teams: Manuel Pellegrini clearly wants to try and get as far as he can in all 4 competitions this season. Whether or not that is wise is another story, but he wants to give it a go, on the chance that this turns into a legendary season. With Man United and Tottenham eliminated from the FA Cup, there are already only a few strong Premier League teams left in the tournament, and a result or two against Arsenal, Liverpool, or Chelsea could make the FA Cup even more obtainable.
It seems as though Pellegrini will play strong teams in the FA Cup, while not going all out. I think he'll wait and see what happens with some of the other big clubs before deciding whether or not to go for it.
For Blackburn, Gary Bowyer has even bigger decisions to make. His team fought well in the first match, but he has to be wondering if it is worth it. His next two matches are against Nottingham Forest and Derby, which are two of the teams ahead of him in the playoff race. Blackburn sit in 8th right now, only two points outside the playoff. Nottingham Forest are 3 points ahead of them, and a win at the weekend would put them in the playoff places.
On the other hand, his team has only played once since the last match (Jan 4th), while City have played twice. He might decide that getting a big scalp is worth a shot. It'll be hard to tell until we see the lineups come in.
Tactical decisions: When I watched the first match between these two sides, it seemed to me that Pellegrini had underestimated the kind of hard-nosed football that would be played. Blackburn played a very disciplined 4-5-1, and really seemed to dominate the midfield, leaving Dzeko and Negredo fairly isolated. They tried to push both of those players out wide, and then collapsed their defensive lines to cut them off from the team.
Fernadinho and Javi Garcia played well last time, but couldn't seem to put together really threatening attacking movements. They did combine well in defense, making sure that Blackburn struggled to find the final ball all match. Neither team was particularly fluid, and both goals came from set pieces.
If it were up to me, I would opt to start the same way that we have lately, with Dzeko and Negredo up front. It has seemed to get slightly better and more fluid each time they play. With Silva and Nasri both out, it will mean Milner on the left, and Navas on the right, with Fernandinho and Javi in the middle. Micah, Kompany, Nastasic, and Clichy will likely start in defense, with Pantilimon in goal.
If we start getting overwhelmed in the midfield, I think that Pellegrini will be quicker to put Aguero in for Dzeko, allowing for another player to drop deeper and help out. I haven't been able to find any information about Jack Rodwell, but he isn't listed as being out. He could be another option to bring off the bench.
Prediction: I think that we continue to dominate at home, with Dzeko and Negredo picking up where they left off from the match against West Ham. Fernandinho will be the key man driving the team forward, pulling the strings with the wide players and the big guys up front. 4-1 for City.