Also a runner here --- I've had runner's knee, which I think is a kind of tendonitis around the knee. In my case -- and I think this is generally how it works -- tendonitis presents as lingering pain that doesn't go away and is aggravated by activity. It's a sign you need to slow down and stop putting the same kind of strain on that area, and possibly also strengthen or stretch in other areas to avoid the kind of imbalanced movements that led to the condition in the first place.
That's my 100% layman's impression of it, anyway.
Biggest concern for me about Jaylen having tendonitis is it means that he needs to have his workload lessened, at a time when the team really needs to lean as much as possible on the stars. But I guess part of the problem is we've been leaning on them too much already.
The other concern is that tendonitis could suggest an increased chance of a more significant injury in that area unless Jaylen adequately reduces the strain and allows it to heal. But as Vermont Green already explained really well, tendonitis can take a long time to resolve. For me, I had lingering pain below my knee for what felt like ages, even when I reduced my running a lot. But it did go away eventually.