Moss and Welker teamed up for 596 receptions, 7,453 yards and 62 touchdowns from 2007 to 2009 before the Patriots traded Moss to the Minnesota Vikings early in the 2010 season.
Welker had more of the catches (346) with Moss supplying more of the touchdowns (47) during their time together.
"Obviously, Randy is a great player and has so much speed and everybody is so afraid of the big play that he can create, and everything like that," Welker said Wednesday on a conference call with Bay Area reporters.
"It definitely puts a lot of teams in two high safeties and kind of opens up the middle of the field."
At 35, Moss has slowed down as he returns to New England on Sunday night to play his former team. Moss has 21 catches for 346 yards and two touchdowns in the 49ers' run-based offense through 13 games.
"I know Randy worked really hard," Welker said. "Even if he did slow down a little bit, he's still really fast. He's been a great playmaker for a long time in this league, and I'm sure it hasn't changed too much."
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Earlier on Wednesday, Patriots coach Bill Belichick complimented Moss, calling him "the greatest deep-ball receiver that's ever played" and "probably the smartest receiver I've ever coached."
After practice Wednesday, Moss spoke to reporters who cover the 49ers. Moss expressed his admiration for Belichick and the Patriots.
"I learned a lot from Bill Belichick," Moss said. "I've said time and time again, before I got to New England, I thought I knew a lot about football. But I think he taught me a lot from A to Z. I still carry it to this day.
"The compliments are great and I respect Coach Belichick and that organization still to this day. And hopefully we have a good game coming out Sunday night. Looking forward to the game, and hopefully the fans do, too."
When asked if there are similarities between the 49ers and Patriots organizations, Moss said, "I don't want to get into all of that because whatever I say is going to be the wrong thing to say. Basically, I'll leave it at, I still got love and respect for the New England Patriots and everything we did as a team. But now I'm a 49er, and hopefully we go out there Sunday night and give it a good game.
"One thing we know, it's going to be a hard-fought game from the offensive side because of their ability to put points up. Hopefully, we can complement our defense, not telling what's going to happen, but we got to play complementary football. And if we do that, and try not to let Tom (Brady) and the offense run the score up on us, I think it'll be a good game. Like I said, we're just coming in and working each and every day and looking forward to the matchup Sunday night."
Moss said he isn’t sure he can provide any useful insight into the Patriots defense for the 49ers’ defensive players. The advice he has given teammates is to be prepared for anything — or else the 49ers could have an experience similar to the Houston Texans, who were drubbed 42-14 on Monday night.
“Everybody knows Bill Belichick is a so-called genius and he changes it up every week,” Moss said. “There's really not much input I can give for the week. I just said, have your head on a swivel and be ready for anything because the way they're playing, coming off a Monday night game against the so-called best team in the league, in the Houston Texans.
“They put 40-some points up and really embarrassed them, so I don't think we want to be that team that gets embarrassed on national television. So like I said, we come prepared and ready to play a good game.”