From what I had heard through the grapevine that off-season, Messier had wanted out. Don't know why, he just wanted out. It showed on the ice, too, during his last season in Edmonton. He missed a considerable part of the season due to injuries, but even when he played, he was not the dominant power forward that won the Hart and Pearson in 1990, and had dominated many years before. He had 15 points in 18 playoff games in 1991 - the worst production pace since his third year in the league.
I don't think Messier even showed up for Oiler training camp in 1991. (Murphy2, Bucky or Ogopogo, can you confirm that for me?)
The Oilers received Bernie Nichols, Steven Rice and Louie Debrusk in exchange. Nichols was, of course, the centre piece. Rice was a young guy a lot of teams liked, a projected second line power forward (think Shayne Corson) or, at worst, a third line grinder. He didn't even develop into that. Debrusk became the fourth line tough guy everyone expected him to be.
To make matters worse, though, Nichols balked at reporting to Edmonton. He liked New York, and didn't want to go to Edmonton. Also, his wife was expecting a child in a couple months. Eventually, he did report, but only after the birth of their child. While he wasn't thrilled to be in Edmonton, Nichols did actually have a pretty good playoff in 1992, centring the Oilers "Pipe Line" with Vincent Damphousse and Joe Murphy. (Murphy likely played the best hockey of his career in the 1992 playoffs). The Oilers made it all the way to the Campbell Conference Final with a relatively young team (I've long maintained that the 1991-92 Oilers could have won multiple Stanley Cups), but were swept by Chicago. All three members of the Pipe Line were gone by the end of the 1992-93 season (Nichols was traded to New Jersey for Zdeno Ciger and Kevin Todd), as was two-way threat Esa Tikannen.
