This is what ive pieced together:
Yzerman's favorite player before he joined the league was Bryan Trottier (as a really young kid it was Bobby Orr and Phil Esposito) and Yzerman has said many times that he tried to model his game after him and he has said during the early 90s - maybe earlier but i havent seen anything. Bryan Trottier is generally perceived as a paragon of a complete player (though i have heard things about Trottier changing his own game and sacrificing his O to become a checking center in his late career) and it is pretty clear that Yzerman has this perception of Trottier too. Interesting note is what Trottier has to say about Yzerman's two way play.
Funny thing is, Steve Yzerman doesn't think he has changed much at all. "When you come into the league as an offensive player," he says, "people think you can't play defense." Trottier, who battled Yzerman for more than a decade before retiring in 1994, knows better. "Who are these people who say that?" Trottier says. "I played against him back then and I remember him killing penalties, hustling and back-checking. I guess no one else does."
Darren Pang who played with him before he went to the OHL has said many times that Yzerman always played the two way game.
DIck Todd who coached him in the OHL has also said the same thing repeatedly.
The Red Wings that Yzerman joined in 84 were comprised of what is characterized a bunch of selfish veterans who had (statistical) incentive-based contracts. Ogrodnick who was Yzerman's winger especially had a reputation like this and there was said to be some beef that Yzerman wasnt passing the puck enough to Ogrodnick. In the summer before the 86 ssn Yzerman lost a significant amount of weight and his play suffered a lot. He was also criticized for floating as well as the rest of the team. Didn't find anything on his early coaches on this except for Polano saying (after the fact) he wanted to make Yzerman captain after 85.
When Jacques Demers came in 87 things turned around. The Red Wings were molded into a physical and defensive team. Yzerman was made captain and by his own words took the captaincy very seriously and was the comeback player of the year. Demers has stated both at the time and after the fact replying to comments about Yzerman's transformation that Yzerman always played well defensively, killed penalties, blocked shots, won key faceoffs. Yzerman for his part has repeatedly said that Demers was the first one to play him in a defensive role in the NHL. Yzerman got a lot of credit for the Red Wings disciplined defensive style that took them to two conference finals. The team started to crumble by 89 and is said to have tuned Demers out and the first thing to go was the tight defensive structure (the Goose Loonies incident is refered to as the starting point but Yzerman has explicitly said otherwise saying that its effect was vastly overrated).
When Bryan Murray came he once again tried to bring a structured defensive style to the team like he did with Washington. He also cut Yzerman's icetime significantly as the Red Wings had Fedorov and Carson at center. By the 2nd half of 91 the Red Wings were playing much more structured team D. 93 somewhat changed that trend. Murray got better wingers like Ciccarelli. Yzerman noted how in 92 he didnt play on an offensive line as compared to 93 with Ciccarelli. Murray had always wanted a superstar defenseman to anchor the D and in 93 he got one but it was Paul Coffey lol... Murray said at the time that Yzerman played both ways, worked as hard on D as any star in the league, and was in general just a complete player. Havent seen Murray make comments on this issue after the fact.
Bowman let the team continue to play offensively in 94 (the D actually suffered) before Barry Smith brought over the left wing lock from Sweden before 95. Bowman has said that he told Yzerman to take a defense first role and sell the system to his teammates that his stats would go down. Bowman has also said that it was easy for Yzerman to make the change and that he was the reason the team changed because it wouldnt have been accepted without his acceptance which goes against notions that Yzerman and Bowman clashed on the D issue. Bowman has also said that the team when he got there was offensively gifted but defensively porus but i have not seen him say anything of the sort about Yzerman's D.
In 95 and 96 Yzerman was playing on a checking line for quite some time with Errey. From 97 he went back to being in the top offensive center role but that was when the rep came. Bowman called him the best two-way player in the league and more blah blah blah. This is also when the anachronistic and simplistic foiling of Yzerman's career began. Yzerman won the Selke in 00, shockingly when he scored 30 goals and was top 10 in scoring (reminds me of Fedorov who lost the Selke in 92 when he was playing a more defensive role but then won in in 94 when Bowman told him to be selfish and play more offensively).
As for some others:
Holland has said Yzerman was one dimensional in his prime.
Devellano has explicitly said Yzerman was bad defensively early on.
Larry Kelly (agent) has disagreed in response to Devellano.
Dave Lewis has been ambivalent saying that there were still a few things to learn.
Barry Smith has said the ability was always there and he was just asked to do it.
(these are all after the fact)
The last thing is that players like Sakic, Lindros, Forsberg were also considered "two way" in their era and even got Selke consideration although they were nowhere near the defensive ability of players like Yzerman, Fedorov, Modano. In Yzerman's era defense seems to be much more of an afterthought for elite offensive players. Yzerman said he wasnt noticed for his D when he was scoring in the late 80s/early 90s but had his prime come 10 years later that might be different.