IMO, since we shifted Gerrard to the holding midfield role and he settled into that position, our central midfield has improved by leaps and bounds. Gerrard was a defensive liability when played in the more forward midfield rank because he didn't have the speed and quickness to press nor stop opponents bursting past him.
The quartet of Henderson, Allen, Coutinho and Sterling individually have a good balance of offensive and defensive skills, which means that they can disrupt plays as well as initiate them. Where Mascherano used to shine for us, is where the quartet play now. He's not a DM in the Makelele or Didi role, but a midfield destroyer who was very good at disrupting opponents' flow. However, we've moved on from that now as our current system requires the forward CMs to be able to press, defend, win the ball and also create. It's the last part where Mascherano falls short, unless he's improved so much at Barca.
Mascherano played a rank deeper where Gerrard plays now would not play to his strengths as he's neither a deep playmaker, nor is he a cool-headed stopper and leader who will hold his position and not go scampering around. So right now, while he'd be a better man-on-man defender than Lucas (by virtue of his better strength, mobility and aggression - he'd probably be giving away as many silly fouls in bad areas), I don't think we get that much more from having him in the team. Gerrard is going to play the bulk of the minutes, and as long as Lucas doesn't play in the forward CM ranks, he's actually pretty serviceable still where Gerrard plays, or as Gerrard's partner when we switch to having 2 CMs at the back for more solidity (bar the brainfade fouls).
I know some say that Mascherano's not as bad a passer as others think, but I cannot shake off that period in my mind during Benitez's final season, when he played Lucas and Mascherano in midfield together. Mascherano ended up being the one ferrying the ball and linking the defence and attack, and we were awful in that period in terms of our attacking play.