The Patriots did not announce the nature or severity of his injury.
With Pittsburgh leading 23-10 during their 33-10 win, Matt Cassel's pass to Welker went incomplete and the receiver was knocked down by Clark, who was penalized 15 yards for unnecessary roughness. New England ended up losing a fumble on that series.
"It was not like I was trying to be cheap," Clark said. "Anybody that comes across [the middle], it is my job to tackle them. The ball got tipped, but [the official] said I should not have left my feet. ... I talked to Kevin Faulk and apologized."
Faulk said, "If Wes Welker stays down, then it must have been a pretty hard hit."
Welker finished with four catches for 30 yards, ending his NFL record streak of 11 games with at least six receptions to start a season. He entered the game with 80 receptions, second in the NFL to Houston's Andre Johnson, who had 81.
Welker tied Cincinnati's T.J. Houshmandzadeh with an NFL-best 112 receptions last season.
Asked about the penalty against Clark, Pittsburgh coach Mike Tomlin said, "I don't comment on penalties or officiating."
Patriots coach Bill Belichick said, "I don't have anything to say about that right now."