There isn't really time to ease Tolvanen into the lineup, especially with another back-to-back coming. They should just put him in whatever spot they think he fits best and let gametime work the kinks out, to get him as ready as he can be. It's actually pretty much the basic maneuver in a situation like this, when a player joins midway through the event.
Not that there's much room for speculation, since they practically have only two spots they can plant Eeli in - either into the first alongside Granlund, knocking Kapanen down to 3rd (which really shouldn't be viewed as "demoting" Kapanen, instead it'd more like trying to jumpstart said unit), or the third next to Suomela, which means Manninen will get the boot from the lineup altogether. If they do that, I can definitely see the justification behind it. After all, the other winger will be either Palola or Jormakka, both of whom were in the same line with Tolvanen in Jokerit - albeit only sporadically.
I would personally just put him next to MiG. Not because I immediately expect wonders from the kid, but I somehow don't think he'd be any worse fit there than Kapanen - and I also think that the 3rd would definitely benefit more from Kappy's presence. Kapanen has actually not been half bad as an individual, but I really don't see the chemistry with the rest of his current unit.