You don't have to play him every day, you only have to play him against RH starters, which is between 2/3 and 3/4 of them.
Marsh is still a good player having a good season, and is one of the better OFs in the NL even. You can't discount his defense either. I'm all for the LF platoon upgrade and a bench upgrade too. A new starting LF who plays everyday would be costly, and also would move Rojas to the bench because even with his defense, you can't play an OF everyday who can't hit, period (and, ironically he has reverse splits so he is not even that much better than Marsh against LHP).
This is still the core of the team that did what it did the last two years and the one that is on pace to win more than 100 games. Panicking or making major changes is not called for. Though, if we do want to panic, I see the 2B spot has now been brought to the floor.