In Diggs first year he primarily play the X WR position which is where Norv Turner kept putting his best WR. Charles Johnson started the year there before they activated Diggs and had him start there for the rest of the 2015 season.
I am sure he played some from the slot and the Z WR position as well. He plays them all, as does Adam Theilen.
In 2016 they did use Diggs out of the slot more.
According to this 40% of his snaps he lined up in the slot. I think he mostly played the Z WR position for the other 60% but he likely lined up at X at times as well.
In 2017 there has been talk about using Adam Theilen out of the slot more and using the above source Diggs has run 15% of his snaps from the slot in the first two games. Not sure about that for the game yesterday.
Adam Theilen lined up as the slot 64% of the time over the first two games, he was used out of the slot 15% of the time in 2016.
These splits are always subject to change any given week because the WR can play all WR positions. So far Theilen is much more likely to line up against the nickle corner than Diggs is.
The Detroit Lions have been playing better defense than expected this year and will be a difficult challenge. I would assume that Diggs draws Slay in coverage for the most part but I don't know how much Detroit is actually playing man coverage or if they would have Slay follow Diggs or not.
I think Diggs can beat any corner in the league. His very first NFL game was against the Broncos and he was beating all of their very good corners pretty easily. Slay may be his biggest challenge this year though.