FORTUNE — Yesterday I spoke with a banker who insisted that Michael Dell and Silver Lake Partners would never get their Dell DELL buyout done at $13.65 per share. He felt that the market had changed too much since the offer was accepted by Dell’s board, as evidenced by Hewlett-Packard’s HPQ stock surge over the past two months.

Not only would the comparison spur shareholders to reject the existing offer, but it also would help rivals like The Blackstone Group (BX) and Carl Icahn secure bank financing for higher bids.

Silver Lake arrived at the $13.65 per share figure on February 4, according to a recent proxy statement. This was after several machinations and outside events, including an earlier Bloomberg report that a deal was in the works.

That day, Hewlett-Packard stock opened at $16.31 per share. Yesterday it opened at $23.73 per share, or nearly 45.5% higher.

Now not even my banker is suggesting that Dell would be trading 45.5% higher than it was on Feb. 4 — particularly given that news of a possible buyout had leaked several weeks earlier. But might it not at least have mirrored HP since before the leak (Jan. 14)? In that case, Dell would have opened today at around $15.71 per share.

Then came word this morning that Goldman Sachs GS had downgraded HP to “sell.” Company shares fell more than 6%, and as of this writing are trading at $21.90. Using the banker’s analogy, Dell also would have been downgraded.

What this means is that HP stock now is up only 33% from where it was before news of a Dell buyout leaked. And if you apply that new premium to Dell, its current stock price would be $14.50 per share.

That’s obviously higher than the current Silver Lake offer of $13.65 per share, and even a bit above the $14.25 per share that Blackstone has indicated (albeit lower than Carl Icahn’s $15 suggestion). But it’s reflective of a downward trajectory, rather than of an upward one.

This could prove problematic for Dell shareholders who are counting on a superior offer, or on large existing shareholders to kill the current deal. If today’s HP experience is just a hiccup, then no big deal. If it becomes the beginning of a slide that lasts a week or more, however, then $13.65 may begin to look like the best possible option.

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