SANTA CLARA — Less than four weeks until the 49ers start their regular season, there is less and less certainty about which wide receivers will be available for that opener in Seattle.

“It’s a lot more open than it’s ever been,” coach Kyle Shanahan said Monday of who’ll make the initial receiving corps.

It’s basically Ricky Pearsall and … fill in the blank. Let’s get right to the roll call of this jumbled unit.

— Brandon Aiyuk is not expected to make his season debut until possibly Week 6 of the regular season, said Shanahan, adding that Aiyuk’s knee rehabilitation certainly should keep him out of the Sept. 7 opener.

— Rookie Jordan Watkins, to his surprise, learned Sunday he sustained a high ankle sprain in the preseason opener and will be out a month, so at least the remaining two pr

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