Saw plenty of the stars during lehtinen's career, thank you. if only stars fans would think he's worthy then good chance he's not worthy absent some really special team-based distinction like, say, a 20 year run like marleau. the fact that the stars wouldn't win the cup without him isn't a case. you can say that about many players. lehtinen had a relatively short career also, for an ostensible jersey-retirement-worthy star. the real question is does it get retied without the cup win? doubt it.
honestly jersey retirement on merit we're talking obvious hall of fame players. modano gets his jersey retired. lehtinen isn't in that conversation.
1) HOF worthiness has never been a pre-requisite for jersey retirements/rings of honor, etc. in any sport. In fact, while HOF'ers are generally 100% shoe-ins for jersey retirement at one organization in their career, jersey retirement seems pretty clearly a way for a team to honor a player who they think is worthy of such recognition, regardless of outside influences.
2) The guy played nearly 1,000 games including the postseason, all while wearing a Dallas Stars sweater.
3) During those games he was a key contributor in winning a Stanley Cup.
4) Three Selkes as a winger.
Your question about whether his jersey gets retired without the Cup win is weird. You say you watched him "plenty" but this argument of yours sounds incredibly reaching. Like, maybe the answer is no, but they did win the Cup. Lehtinen played a big part in doing so. So that being part of the criteria is justified.
14 seasons, 1 Cup, 3 Selkes, and the embodiment of what Dallas Stars hockey was all about during their heyday is a good enough resume for me.