As Spain said, bringing Lennon back into the fold is a must. No idea what MP has against one of our most reliable performers in EPl history. True, he isn't world-class and doesn't score enough goals. But he can stretch a defense so that a CB drifts wide to help his FB deal with the pace and dribbling ability (both sorely lacking in Lamela), thus exposing gaps through the middle for Kane and Eriksen to exploit.
I sincerely hope that Pritchard will be the answer to this question. A good friend of mine is a season ticket holder at Brentford and he was absolutely raving about him the entire season.
But if he's not yet ready to step up a notch, you've got a very good question. Although the exact same question can be asked in reverse: with the inevitable Schneiderlin-not-going-to-spurs now transpiring, who would you go for for the DM position? If anything, that's an even harder question to answer.
I'd say even Mirallas is a step up from Lamela tbh. The biggest problem with playing a 2-3-1 as we do is that 'the 3' are exceptionally easy to defend against. Any well-drilled team will have their CM's watch out for late runners or overlapping full backs (who in turn need the pace of Walker, Rose or Yedlin to work), and have their FB's slot in next to their CB's in central positions to essentially make playing anywhere within 30 yards of the goal like trying to have a kickabout at Kings Cross during rush hour.
The stupidity of a system without width and pace to stretch the FB's away from their CB's is that you basically need your '3' to be as good as Hazard, Willian, Oscar etc. We currently only have one player in that bracket of class - Eriksen. The rest of our midfield are sadly a poor man's Chelsea, which is an insult of the highest order.
In Kane, I belive we've finally found a player perfect for the '1'. Now we owe it to his continued development to invest in the '3'.