Steele said so in the interview below. He said tightness and pain.
Also, tendonitis is what the Cubs said it was, but this was announced before he even had an MRI, so them saying "tendonitis" is more of a guess, probably because he says its the same thing he felt last Sept when he went to the IL, which they at the time diagnosed as "left elbow flexor tendinitis", but I believe that was also before he had an MRI in Sept.
Key is that the UCL is a ligament, not a tendon, so a UCL could not have "tendonitis". However, note that Alzolay also was diagnosed with "right flexor strain" in May 2024 when he went to the IL but ended up having TJS a few months later, so what the Cubs say may not be 100% accurate.