Out of curiosity, what have you actually seen from Grezda that gives an indication that he has ability?
What I've seen is a player who looks awkward on the ball, lacks composure, and has been bypassed by other team mates reluctant to pass him the ball.
There's been plenty of occasions when he's been fit and Gerrard just will not play him, or will only give him very limited minutes. He'll get about 15 minutes if we're cruising - at best.
The bulk of the evidence points towards him not being able to cut it. Whether other players have looked poor and then turned out to be effective elsewhere is neither here nor there as firstly, they are the exception rather than the rule. Secondly, we can't waste more time and wages on a player on the off chance he'll come good (when there's little evidence to suggest he will).
This guy is on healthy wages and was bought for a substantial fee. It's really not too much to ask for him to show a helluva lot more than he has.