With Rakell carrying the puck down the right wing, Lindgren stepped up and delivered the check while Rakell had his head down. Rakell's head pops back upon contact, and shortly after he left for the Penguins dressing room.
With another view, the check seems clean. Lindgren glides toward Rakell coming down the wing, and the principle point of contact is the chest. The check being forceful or causing injury shouldn't dictate the penalty, but only whether the check was legal or illegal.
The Penguins will certainly wish for a quick recovery for Richard Rakell, being their premier trade deadline acquisition. On the other hand, the Rangers have set a physical tone to start the series. It's shaping up to be some classic playoff hockey. Rakell did not return for the 2nd period.