My opinion on Lucas is that he is a good, steady player. I'm going to state what I believe are the positives and negatives of Lucas.
Positives: Neat passes (forwards, backwards and sideways) with a high percentage completion rate. He's not the best tackler, but at least he gets stuck in now. He is a hard worker and puts his all into every game. He has a strong mentality, and has improved as a player despite heavy criticism.
Negatives: He is poor in the final third, which is justified by his lack of goals and assists. His assists for a central midfielder is way below average. He doesn't really thrive in the final third. IF he contributed to our attack, he would get alot more praise, but because he doesn't, he is seen as a bit of a 'drifter' by many.
Now, how good is Lucas? Would he be part of a team aspiring for top 4 or a title? Would the likes of Barcelona, Madrid, Chelsea, Man Utd, Man City, Chelsea want Lucas in their team? In my opinion, the answer is no. I think Lucas is a decent little player, BUT, if we are aspiring in the future to be challenging for the title, Lucas should be a substitute at most.
As for people stating he is average? Well, they may have a point. He is not the greatest defensively, and is actually below average going forward. His strong points are picking up the ball, making a neat pass and keeping the ball moving in the centre of the park. He is above average in that aspect, but below average in others, which kind of evens him out to being, well, average.