In the production blog it clearly stated "There was some scrambling during my first day on set because we got a phone call telling us that Tim Omundson, who plays Lassiter, broke his collarbone while mountain biking."

Mystery solved.

Good to know! "Hope your arm gets better Tim!"

Ahhh, good job on clearing that up!
I hope he gets better soon

Considering these episodes were shot weeks, if not months ago, Tim should be better by now (unless he did some serious damage). I'm guessing he probably has the sling for at least one or two more episodes which would make for a great mini-mystery where Shawn and the viewers get little clues throughout the episodes leading to some hilarious, uncharacteristic cause of Lassiter's injury. (Maybe he went fishing with Henry again and after the first ten minutes decided that his easiest and least painful way out was to injure himself)

I like Lassie!! He reminds me of Felix on the Odd Couple!! He's square, yet funny. I enjoy the love-hate thing with him and Shawn!!

Or he decided to make a dive into the water to escape but missed and his landing wasn't the water? Which would explain your theory.

Precisely what I thought as well.

Thanks for letting us know! That was kind of confusing.