I genuinely think we signed him on a whim because Ajax offered him to us and someone in a suit thought that's a good price, and nobody at the club had the nous or authority to stop the deal.
This has been mentioned before - but Donny was apparently on a list of No 10s (more or less) that we considered the summer before Bruno arrived. As in - he was an alternative to Bruno, it would seem, at that point. We then signed nobody in said position, for various reasons - and then, in January, we finally signed Bruno.
And
then - yeah - we went ahead and signed Donny too the following window.
One theory is that we had already progressed pretty far in talks with Ajax/Donny/his people at an earlier point (going back to the previous summer, I guess) - and since we couldn't get a deal done with a CM (at least not anyone Ole wanted), we picked up Donny.
So - yeah - makes no sense as a transfer for a serious football club, but for Ed Woodward it's less shocking, I suppose.