Sergei Gonchar had 27 points in 45 regular season games as well as six points in 10 playoff games. A total of 33 points in 55 total games projects out to 49 points over an 82 game season. While the team's power play in the regular season struggled, it would have been even worse without Gonchar's team leading 12 power play points. His 24 total assists were seven more than anyone else on the club, and overall his 0.60 points/game were tied with Kyle Turris for the club lead among the players that played most of the season.
Offensively, there is no doubt he had a very good season, but how did he do defensively? Looking at shots attempts among the six defenceman that played most of the season, he had had the third best corsi relative on the team (-1.3) but also played the third easiest competition and started in the offensive zone more than anyone except Patrick Wiercioch. He wasn't getting killed out there, but he wasn't exactly a rock either. At this point in his career, he's an average defensive player with the ability to contribute offensively.
Erik Karlsson's absence did lead to Gonchar receiving 37 seconds more of power play time per game than he received last season and a healthy Karlsson should see Gonchar's power play ice time fall back down. However, if Gonchar leaves, the Senators have only Karlsson and Wiercioch with enough offensive skill to legitimately warrant power play time. This can be mitigated by having Karlsson play the entire power play and having a forward man the point on the second unit, though the latter isn't something that Paul MacLean has chosen to do so far. The more likely scenario is that Jared Cowen or Andre Benoit (if he returns) will play on the second unit, which is a less than ideal solution.
Gonchar's presence 5-on-5 will also be difficult to replace internally. Jared Cowen played sheltered minutes as a rookie and barely played this season. Expecting Cowen and Wiercioch (or Eric Gryba) to be an effective second pairing is likely counting on too much at this moment in time. Given MacLean's propensity to protect Wiercioch in the regular season and bench him in the playoffs, it doesn't seem likely he would want to go into the season counting on Wiercioch playing in the top four.
The truly ideal solution would be obtain a legitimate top pairing defenceman to play with Karlsson and then throwing Methot-Cowen out for the defensive zone situations, allowing Karlsson the offensive situations that he can exploit. Acquiring a first pairing defenceman is difficult, so the more realistic approach would be to find a #3 defenceman that can play with Cowen, whether that is Gonchar or not.
A bottom four of Cowen, Wiercioch, Gryba and Phillips would not inspire a whole lot of confidence in a Senators defence that allowed the eighth most shots in the NHL this season and second most in 2011-2012. Purely from a hockey perspective, keeping Gonchar is better than trying to replace him internally. However, Gonchar has said that he would like a two-year contract. Is that a term that the Senators would feel comfortable committing to a 39-year old? Given his offensive production, there is a team out there that doesn't have an Erik Karlsson that will likely give him the two years that he desires. It is unlikely the Senators would commit to such a length unless Gonchar takes a significant cut from his current $5 million a year salary. While the coach has repeatedly praised Gonchar throughout the season, this might end up being something the coach and the general manager disagree on. Bryan Murray will likely do his best to sell one year to Gonchar, and perhaps another year at $5 million would be enough to convince Gonchar to return. If not, there is a good chance that the Senators will be active in the free agent or trade market.