He's certainly been underperforming this season with respect to his potential and how he performed last season, which is absolutely no great crime since he's 21 years of age. He's got an unreal amount of talent, technically he is absolutely superb, but not much is coming off for him at the moment. It's less of a problem now Sturridge is back, because hopefully he can now benefit from a bit of a rest, as I think people underestimate the effect it has on a young player to overuse them through poor form or difficult times when not much is falling for them (see Jordan Henderson in his first season here). But yeah, his talent is absolutely not in doubt, he'll be a massive player for us in the future.
Agree with some above, that I think there are probably two main concerns with respect to Phil, though.
1) His position. His "ideal" position is in the number 10, but to be honest I don't see when he will ever be used there. We usually play with a three in midfield, and as long as Gerrard is one of them I don't think Phil can operate as another. More to the point, if we play our preferred version of 4-3-3, there isn't really a nominal number 10, and even the furthest forward midfielder is expected, to some extent, to be box-to-box, which Coutinho isn't really. If we played more of a 4-2-3-1 then it might be different, but we don't. If he plays on the left, then it curbs his natural game somewhat and blunts his effectiveness. He doesn't really provide enough end-product to be playing in that position on a consistent basis, certainly at the moment. Now, if we played two up top with a player in behind them then Phil would be the perfect candidate, but I don't really see him as an ideal fit in any other formation we play regularly. Certainly not as a left-sided midfielder in any version of 4-4-2. He's great at applying pressure, but tracking back really isn't his game.
2) His shooting. It will improve, and he's snatching at things at the moment, but it's still pretty poor. It's not just that he doesn't finish well, it's his decision making when he's taking shots that's flawed at the moment. I'm all for "buying a ticket" but it disrupts our game sometimes. You only need to look at Suarez to feel confident he can improve, but it's a big problem at the moment.
Ultimately, I doubt we'll play with all four of our forward players at once, so at the moment, Sterling probably edges out Coutinho for that third attacking spot, but a lot will depend on how we set up going forward. Make no mistake though, Coutinho will be obscenely good in time.