Wasn't he good for most of the series especially game 5?
No, he was a complete no-show until that game. Pretty sure those were his first points of the series.
If we're picking "disappointing" Blues, I'd say Brandon Saad. He scored 8 points in 10 games for the Avs last postseason, but seven of those were goals. He put up 5 points in 12 games this time around for STL, only two of those were goals, and IIRC, one of them was an empty-netter.
Runner up...Pavel Buchnevich, of whom I've been a fan of since his Rangers days, had an absolute monster season for the Blues, but then posted 1 goal in 12 games in the postseason. Mind you, he put up 10 points in 11 games, but when only one of those is a goal, you expect more for a top-flight two-way player than that. The only time he really stood out in the series against the Avs was when he and David Perron were taking concurrent crross-checking penalties in game four.
Kyrou had a pretty good playoffs and series against the Avs, but hitting Manson instead of the net when it was 2-1 Blues in the 2nd, and then missing the net on a clear breakaway with a minute to go in the 2nd were huge factors in their loss.
I thought I read somewhere Kyrou missed a defensive assignment on the eventual series-clinching goal. Talented as he is, his defensive game still leaves a lot to be desired, and his production hasn't yet reached a level where one can simply ignore those shortcomings. Really good player, but he has his flaws.