I've seen him a lot this season (in-laws are season ticket holders and my son is a Boro fan)
He was far and away Boro's best and most effective player until he got injured and often had teams tripling up to mark him out of games. Which was effective as Boro had no threat down the other wing at all. Despite this, he still had 10 goal involvements in 24 games plus many other chances created that were missed or got into good positions and didn't capitalise. This doesn't overly concern me, as Morgan Rogers was extremely inconsistent at Boro then hit the ground running at Villa and looked a completely different player.
Doak is also brilliantly two footed, which is rare these days. He can go either way (to the byline or across the box) when taking his man on and scored a lovely left footed shot from the edge of the box last season.
My concern would be the injury and £30m is Liverpool tax for a wholly untested player in the Premier League, but after I was screaming for us to get Gyokeres from Coventry after watching him destroy teams all season and easily be the best player I'd seen live that season in the Championship, I'd be happy with a deal around £20m + performance based add ons