Sunday's Premier League clash against Manchester City took place with the Terriers closely linked to the Borussia Dortmund II coach, who was the hot favourite to succeed David Wagner.
And for a few brief minutes it appeared that broadcasters Sky Sports had spotted Siewert in the home stands as they went down 3-0 to remain bottom of the standings.
It proved to be a case of mistaken identity, however: 'Siewert' turned out to be Martin Warhurst, a local hospice director who happened to be sitting in the Huddersfield directors' box.
Siewert was eventually presented as Huddersfield's new head coach the following day, with a very special guest present in the club's reveal.
The video shows Martin occupying the Terriers hotseat, complete with a club jacket emblazoned with his new initials, MFW, before Siewert 'ousts' him and takes over.
And in all fairness, the club's new celebrity has been an excellent sport about the whole episode.
"It was bizarre. Basically what happened is I was sat in the crowd and suddenly I was aware of a guy coming towards me from the right-hand side," he explained to Sky Sports.
"He said, 'Are you Jan, the new manager?' I laughed and said, 'No, no, that's not me. I'm Martin from Wakefield'.
"It was absolutely bizarre. I thought the guy who came over to me asking if I was Jan was going to say something like, 'Excuse me, sir, but you're sat in the wrong seat'. I had to double take when he asked me if I was Jan.
"That was all I heard of it and then suddenly everybody's phones and my phone started going crazy, saying, 'I've just seen you on telly'.
"There was lots of reaction from people in the crowd - just people coming up and having selfies and people patting me on the back and wishing me luck.
"I was on my way back up to the table where we were being hosted and a father and son stopped me and the dad said, 'This is the new manager'.
"A couple of the girls next to me had pulled up pictures online to search what he looked like and they did a split screen thing. There is a bit of a resemblance so I can sort of understand where they're coming from - obviously I'm a much more attractive guy, though!
"I will be following him more closely from now on - my doppelganger. I had no idea about him, I didn't know who the club had been linked with."