Tielemans just does things that are nearly impossible to replace. There was an early ball, a free-kick I think, where Dougie tried to dink it through the right channel of the penalty area, and just about didn't get it right. It was a good effort - but it's something Tielemans hits pretty much every time, because he's incredible.
Even more so though, what was that insane statistic about Tielemans having the most line breaking passes in the league by miles? Turning defence into attack, beating presses and releasing Rogers or McGinn, conjuring up space from nowhere - Tielemans is literally as good as anyone on earth at doing that. It's tough on Dougie, having been through incoherence at Juve and unfitness at Forest (not to mention playing for cloggers), to suddenly be dropped into those boots.
He's a good player, does many many good things. He just can't replace Youri, because pretty much no-one can.