In speaking on skating with Matthews, Knies may have accidentally revealed that Matthews suffered an injury while training. Knies said he met Matthews, but he wasn't skating at the time due to "an injury or something".
Knies was very careful in his wording as he prefaced his statement with "I think" as he was not entirely sure. Given that Matthews was playing through a lower body injury during round 1 against Tampa, and it had held him out of the final regular season games, it doesn't come as a surprise that he wasn't skating consistently earlier this summer.
Matthews has been seen on the ice consistently the past month or so down in Arizona so it looks like he is fully healed from whatever injury Knies is referring to in his interview, which is good news for the Leafs. With training camp approaching, Matthews should be making his way back to Toronto in the coming weeks to get some informal skates in with other Leafs in preparation for camp.