Ricketts is well below average for a starter and was disastrous last night, but it's worth pointing out that the Galaxy have been struggling for a little while now, even in games that Ricketts doesn't give away. I think they have larger problems than just GK, which revolve around the fact that Keane, dos Santos, and Gerrard are not fitting all that well together. All three are different players, but they all want to play somewhere in the spectrum between a defensive midfielder and a forward who leads the line.
There aren't enough positions in that zone to accommodate all three, however, which is a problem. Keane has been so effective for LAG that he's always going to play in the same position. Dos Santos can't be played out wide because he plays zero defense, so he's Keane's partner up top by default. However, they are both forwards who like to play underneath, which disrupts the balance of the attack. Zardes has to be shunted out wide, which isn't his best position.
In the midfield, Gerrard has looked like a 35 year old who hasn't had time off in a long time -- the summer of 2013, really, given the 2014 WC and the 2015 move to MLS. Down the stretch he's been notorious for not tracking back, not tracking runners, not being very mobile. He hasn't been Pirlo-level useless on defense, but he's been a liability. LA's defensive pairing of Omar Gonzalez and Leonardo is great at cleaning up crosses or operating in a compact defense, but they aren't the guys you want when the other team has runners free in the box or is able to create 1v1s.
Balancing this team has been a struggle and it's one that Arena still hasn't been able to resolve. On some level it's a similar problem to what NYCFC faces -- in MLS carrying multiple luxury players is quite difficult.