Seems like there isn't a lot of consensus about his pre-season play but I do see that he meshed well with high-skill guys (which is awesome, he's got offensive talent to feed and convert in those spots) and also had some of his disengagement in a game.
Just a caveat about the O'Reilly comments above - his play in St. Louis wasn't "flipping a switch", it was the same way he played in Buffalo. He did the same things there he did with the Blues (metric-heads can attest to most of that), but without playing for a dufus HC and perhaps the stress of his first child being born (something from mid-season in his final year in Buffalo). In terms of on-ice performance, he was pretty much as advertised in both cities.
Which, coming back around to Nylander and a bit about Jokiharju, both seem to have had the good and bad parts of their respective games on display in preseason.