It is too early to say what the loss of Jeff on Coupling will mean to the dynamic of the show. It is impossible to think though that a show like Friends – which Coupling is so based on – ever losing one of their cast.
On the pro-Jeff front, he was easily the funniest of the (male?) characters. The surrealist who strangely logical flights of fancy were often high points of the episode. HE was the inexperienced, rubbish at it guy who thought he knew all the rules. Richard Foyle who played him also looked funny and had a Welsh accent which stood out in the otherwise bland estuary middle class set.
On the anti-Jeff front, he was the character whose story arc could only make him less funny. He could not continue to be rubbish. And indeed he did not, the series progressed with him becoming on the whole more confident. Steven Moffat was very good at devising more farcical embarrassing realms for him to stumble into, but without development the character was increasingly turning the same trick.
Whether replacing him with a similarish farce happy character was a good idea remains to be seen. The episode on BBC3 on Monday was still pretty good (actually it was a masterful phone episode). The reveal with the new character just felt weird – and this sci-fi geek has not been distinctive enough yet. But Coupling can survive, it is written well enough. Though as Jack Davenport insisted on Monday – ‘This is not an American sitcom’.