Last season he played his last season in a youth team at Wuppertaler SV Borussia U19 and it is fair to say that it was an awful season for the club and his goalkeeper. The club conceded 92 goal in a 14 teams league and scored only 20. Only once Tim Boss had a clean sheet when Wuppertal played 0:0 against Bonner SC. So it is no surprise that his Goalimpact starts off very low. He still has too few games to differentiate significantly from the other players of his club.
He struggled a bit to find a club to play for until he signed for Fortuna Düsseldorf. The season didn't start off very promising in Düsseldorf either with 8 defeats in the first 10 games, but Tim was mostly on the bench in these games. He started playing as number one regularly in November and Düsseldorf lost only 2 games since then, drawing four and winning five. And according to local media, Tim secured this performance by some very good savings. In six games Tim didn't concede any goal. His Goalimpact increased accordingly.
His performance is still below the average of his team mates, but the gap is closing. In March his Goalimpact increased from 60.7 to 66.9. Not quite there if he intends to play Bundesliga (>100) or 2. Bundesliga (>90) one day, but on a good way to settle in Regionalliga and at least get the chance to improve further.
I found a novel way to estimate the one-year rolling performance of the players. For Tim this looks like this.
The new dark blue line shows the estimated Goalimpact of the rolling previous twelve months. It starts a bit later than the other curves as before October 2012 I don't have a full year of history. It starts with a value of 46.7. This is the estimate average Goalimpact of Tim Boss in the time interval between 1st of October 2011 and 1st of October 2012. Notice that this value is below the career average Goalimpact in that time, because the career average is corrected for the regression to the mean. It already assumed that part of the performance of Wuppertaler SV was bad luck and not only missing skill.
Since October 2012 the rolling 1Y Goalimpact of Tim just went North and is now above the career average. Consequently the career average increased too, but not as much as it now assumes that part of the strong performance is luck. It will increase further as long the strong performance persists as luck can be ruled out more and more. As now still some of the bad months at Wuppertal are within the 12 month rolling window, we can expect further increases at least until the summer.