Pace, power, technique and athleticism.
Watching Chelsea v's Utd (for my sins) and seeing a player like Matic showed the blue print for the right type of player for the Premiership. When Chelsea brought on Mikel it co-incided with Utd getting more of the ball and pushing Chelsea back, the difference? He is slow, he can't get up and down the pitch so the team retreated to compensate for that. It's the same with Gerrard, for all his excellence on the ball, I think if our defense wasn't a bunch of headless chicken he has to babysit we'd see far less of him on the field, as he simply hasn't got the legs to compete v's much younger (less skilled) players.
It's why I think Allen for all his industry, technique simply won't cut it for us if we aspire to push on towards the title. He gets knocked off the ball far too easily, just like Lallana - it's pointless having all the quick feet in the world if 9 times out of 10 you simply get bullied off the ball by a huge clogger who hasn't got even the slightest amount of skill as you.
We seem to have players who for the most part do 1 aspect of what is a great midfielder.. This is a slight hyperbole to prove a point, so bare with me.
Henderson - Athletic
Allen - Technical
Gerrard - Vision/Leadership
Lucas - Reading of the Game
Players who more "specialise" in a certain area than being an all rounded player - in this day and age and in the Premier League in world football you can't just have 1 cravat to your game, you need it all - it's one of the reasons I think our defence suffers, our midfield isn't good enough - we are too slow/static and don't cover the ground in front of the midfield properly, we get cut open with pretty simple passing at times, we don't track runners well. If we slow our very soft center our defence will grow in stature I'm sure. At the moment it appears a vicious cycle of needing Gerrard's leadership as our backline are displaying none, but Gerrard isn't covering our midfield due to his energy levels so the backline suffers.
Can gives me great hope as he has good feet, a great turn of pace and is a big strong lad, he fits the profile.. If Henderson could unleash his shackles and stop playing it so safe he is another player who has all the correct attributes etc.
I believe the midfield has moved away from specialists - Germany in the world cup had Kroos, Schweinsteiger and Khedira - you couldn't say which of any's "weak" area of their game was, as they don't have one. Any one of the 3 could drop deep and dictate play or cover the defence, all had the engine to get up and support the attack, they could all pass long and short and score.. It's the future of the game. The days of the "Makelele" specialist are a thing of the past, why have 1 excellent defensive midfielder when you can have 3, but those 3 can do all other aspects of the game also?
Liverpool are stuck with specialists who individually are excellent at certain aspects of the game, but not all round excellent. So if one of them is out we loose their individual excellence in that area and are always weaker as a whole. When Allen is out we are screaming for a player who can take the ball forward and keep it in tight spaces, when Gerrard is out we loose the ability to switch play and his leadership, when Henderson is out we miss his pressing and energy across the park etc.
We need to move with the times or get left behind.
To answer the OP?
I'd play Can + Henderson + Coutinho, with a direct Gerrard replacement my primary aim for January/Summer.