Focussing on their league form is perhaps the most straight forward way to try and answer that question.
There is no doubt he has steadied the Dons ship, a ship which looked like it was sinking earlier in the campaign.
In their opening six games – against Hearts, Celtic, Falkirk, Livingston, Dundee United and Motherwell – they mustered just one point.
They also conceded nine goals, and failed to score in any of those fixtures.
But after that horror half dozen, they have gone on a run which has seen them lose just one of their following five league games – against Dundee, St Mirren, Hibernian, Kilmarnock and Motherwell – a sequence which can be broken down into three wins, a draw and one defeat.
They have managed to shut the door at the back too, only conceding three goals, and scoring eight.
So a clear improvement which has seen them edge up the table to eighth, where they are very much still in touch with the teams above them.
So that all sounds fairly positive, but there remain clear causes for concern.
Creating goal-scoring opportunities, and taking the few that come along, remains a huge problem.
While Aberdeen boast the fourth best defensive record in the division, they also have the joint worst goal scoring record.
Speaking in the aftermath of Sunday’s draw with Motherwell, Thelin noted: “We have some good opportunities, at least we are there and again if you are stable in the defence you can always grow.
“But still we have to improve the way we attack with the ball because when we get the ball with control to our front three we are always dangerous but we need to do it with more consistency.”