The McClement is broken up virtually every game. That is done to allow proper ice time (as determined by the coach). Jay gets "full" "checking line" time (10-12 min.), plus he gets 5 on 5 and 4 on 4 second and 3rd "scoring line" or "2-way line" time with more capable scorers (4-6 min). He gets some PK time, and, once in a great while, gets a shift on the 2nd PP unit.
Often, when there's a Top 6 injury, he moves into the 2nd scoring line for the main portion of games.
He did well with Backes and Stempy. He can score and dish the puck. But, The Blues need a main "Shut Down" line. I wouldn't feel comfortable with Sobotka centering Crombeen and King/Drazenovic/Porter or an average checker, as a "checking line" to play major minutes against other teams' Top scoring lines. The Checking line centre has major defensive responsibilities. He's got to keep the opponent centre from being involved in and setting up offensive plays. He's got to shadow his man. He's got to know how to use his body to impede him. He's got to react very quickly, anticipate well, see the ice well (recognise developing plays), and know good defensive positioning. He's got to be an excellent pokechecker and be able to steal the puck often (especially if he's not a big hitter).
According to Bruin fans on this thread, Sobotka is NOT that type of player. That is why he's gotten almost no PK time. It sounds to me, based on the description of his skills set, that Sobotka COULD become a good defensive player. He has good hockey instincts. Given that, defensive skills CAN be learned. Let's hope that Payne and Mellanby can get him to learn and develop those skills, so he can be a more valuable cog in this team's machine.