This squad isn't really lacking in the DM position in my opinion, with Henderson, Can, Lucas, and Stewart all capable of playing there.
Looking at the bigger picture, the first XI of Henderson-Lallana-Wijnaldum has bossed plenty of midfields - but the problems this season have generally come when that trio has been tired or 1-2 have been unfit.
The solution against the teams that defend deep isn't really to add a better DM (i.e. replacing Lucas), but rather to add a top creative midfielder who can play next to Lallana in those games (or instead of him when he's unavailable) since the midfield often lacks dynamism when Lallana isn't playing.
For example, someone like James Rodriguez would be a great option, capable of playing in the CM (in rotation with Wijnaldum and Lallana), as well as covering at RW when Mané is unavailable. Two birds with one stone right there.
Henderson
Lucas
James Lallana
Wijnaldum Can
That's a pretty strong midfield 6 right there, including Lucas as DM cover. Klopp kept Kehl at Dortmund until age 35 so why not Lucas? A great character and veteran to have in the squad especially given his Champions League experience.