SAN FRANCISCO — Of the 37,000-plus people at AT&T Park on Saturday night, only one fully appreciated what Buster Posey was going through.

It wasn't Buster Posey.

Oh, he'll gain clarity soon enough. Posey made his much anticipated season debut for the Giants on Saturday, with three run-scoring singles in the Giants' 12-1 throttling of the Arizona Diamondbacks. It wasn't his major league debut — he played sparingly as a September call-up last season. But it was his first experience as a player expected to do heavy lifting for a big-league team with playoff aspirations.

Things are probably moving pretty fast for him right about now. Matt Williams could tell him all about that, because once upon a time he was the guy in the fish bowl.

"It's the fulfillment of a life-long dream," said Williams, Arizona's first base coach, recalling his early days as a Giants player. "It's a little scary at times, depending on the situation. But I would imagine that he is excited and ready to start his big-league career and hopefully never gets sent down again."

Giants history 101: Williams was the last position player drafted and developed by the Giants to appear in an All-Star Game. Selected with the third pick in the 1986 draft, he was a slugging shortstop who scouts projected as a major league third baseman. His arm was live, and his bat was supercharged.

He was who Buster Posey is now, a can't-miss prospect with only one question mark: When would he be ready? We know how that debate sounds from the outside looking in. Williams knows what it's like to hear it in your own head.

"Regardless of whether you feel you're ready or not, you need to perform," he said. "This is the highest level. It's all about what you can do. Some guys handle it great. Some guys, it's pretty evident they're not ready."

Giants fans, who have been clamoring for Posey's promotion all spring, believe his arrival is overdue. Twenty-three years ago, Williams became the cautionary tale for throwing a can't-miss prospect to the carnivores before he's ready.

"(Jose Uribe) pulled a hamstring in L.A., a play at the plate or something, about five games into the (1987) season," Williams said. "At that point I had 70 games of A ball and that was it. Clearly I wasn't ready."

Williams hit .094 in his first 10 major league games. He was sent down the first week of July. He played well enough at Phoenix—then the Giants' Triple-A affiliate—to earn a recall in September.

He was rushed to the majors again in 1988, this time when Uribe was placed on the bereavement list after his wife died during child birth. Williams hit a grand slam off Nolan Ryan in his second game back—"Swing hard in case you hit it," he said, smiling at the memory—but was sent back down in mid-June and recalled two months later.

He started 1989 in San Francisco, was sent down in May, and recalled—to stay—in late July.

"Often times that's part of the deal," he said before Saturday's game. "And he's switching positions, too? It's not easy."

The position switch is something else with which Williams can empathize. Posey will be a full-time catcher soon enough. Williams played shortstop early on before moving to third base.

If it can be argued the Giants have been exceedingly cautious with Posey this season, it can also be argued that they promoted him under fortuitous circumstances. For starters, Arizona's starting pitcher Saturday was Billy Buckner, whom opposing hitters were abusing for a .361 average. Sure enough, Posey touched Buckner for two of his three hits.

Secondly, Posey arrived with the Giants having won three of their past four, scoring five runs in back-to-back games. No savoir needed here, just a contributor.

Thirdly, the team has left his status somewhat open-ended. It's entirely possible he could go back to Fresno when Edgar Renteria comes off the disabled list. But no one in the Giants camp would cry if he hit so well they were forced to make a spot for him in San Francisco.

Williams would know all about that dynamic, having been Buster Posey once upon a time. What the Giants would like, at long last, is a position player who knows what it was like to be Matt Williams.