While it's unfair to expect Manning to be a top tier quarterback in his second season in the league (his first full one as a starter), the Giants need him to be that now.
Success in the NFL is not reliable from year to year (ask the Eagles, Falcons or Jets), and right now if Eli were what Tom Brady were in his second year, or even what Jake Delhomme was in his first season as a starter with Carolina, the Giants could come out of the NFC, and who knows what would happen in the Super Bowl. This team has a great pass rush, a great running back, good weapons at receiver (assuming Eli can throw it accurately to them) and an All-Pro tight end. The offensive line has exceeded expectations, and if the linebackers can get healthy for the playoffs, this team (with a good quarterback) would be dangerous. If Eli plays like he has the last couple weeks, the Giants might win a home game, but that's it.
Michael Strahan and Tiki Barber, two of the keys to the Giants this year, aren't getting any younger. The NFC is beatable - the Giants showed they can beat Seattle on the road (losing by three in OT despite 16 penalties and three missed FGs). But Eli has to play better, and he has to do it right away. Sorry for the expectations, but they gave up a lot for him, and just like other quarterbacks who surpassed expectations early on from Brady to Kurt Warner, Eli needs to get it done now.