First of all, on the strikeout issue...Janssen struck out 8.22 batters per 9 innings over his time in the minors. He's not a terrible strikeout pitcher though I wouldn't expect huge numbers there from him.
More importantly though he showed outstanding control with a 1.12 bb/9 and he does a good job of getting ground balls which allowed him to only give up 0.37 hr/9 in the minors.
In the major leagues while the strikeouts are down the rest of his numbers are right where you would expect them to be. He's put up a k/9 of 4.37 but with a bb/9 of 1.29 and a hr/9 of 0.65. While he's been a little lucky on the number of home runs given up (a park adjusted hr/f of 8.4%) even if we adjust that up to 11% that would still give him a hr/9 of 0.85 which is very good.
Using the adjusted hr/9 we can expect an era of 3.89 and a whip of 1.244. I would consider that a WORST case with Janssen as his 57% ground ball percentage mitigates his extra base hit risk while he's always shown great control which lowers the risk for big innings. Those skills aren't going to deteriorate and since the era/whip projections are based off of those skills the only variable is his strikeout rate. Given that he's striking out batters at nearly half the rate he did in the minors I think there's quite a bit of room for growth and improvement here. I don't think we can ever expect Janssen to get back to his minor league numbers - there's a reason those hitters are in the minor leagues - but I don't think it would be unreasonable to expect about 6 k/9 out of Janssen. If he can get his numbers to that point his expected era drops to 3.52 and his expected whip to 1.171.
Disclaimer time...Janssen is a young pitcher coming out of college and was only a 4th round pick. He could certainly be playing over his head and has the equivalent of one full season in TOTAL pro experience. It's a very small sample size to analyze and he could slip back easily. But the numbers he's put up to this point indicate a solid fantasy option that could develop into a very good SP. The lack of k's are certainly a concern in some formats but in real life even his terrible k/9 at the major league level this year can sustain him as a successful pitcher since he gives up so few walks and extra base hits.
Hope springs eternal
grows like ivy on the vine
then dies in the fall
--Haiku for a Cubs Fan, The Loveable Losers