Keller seriously injured his right knee during Saturday's preseason game at Houston, two people familiar with the diagnosis said Sunday. They confirmed the extent of the injury on condition of anonymity because the Dolphins made no public comment.
Keller signed a $4.25 million, one-year contract with the Dolphins in March after five seasons with the New York Jets. His injury represents a significant setback for the Dolphins, who acquired Keller to replace tight end Tony Fasano and help second-year quarterback Ryan Tannehill.
Miami ranked 30th in the NFL last year with 13 touchdown passes, but Tannehill and Keller had clicked in training camp and connected on a 22-yard touchdown pass in the second exhibition game.
One option to replace Keller is Charles Clay, who has made 34 receptions in two seasons while dividing his time between tight end and fullback. Potential replacements also include fourth-round draft pick Dion Sims, who has had a good camp, and Michael Egnew, who played in only two games last year after being drafted in the third round.
"We have other guys who have caught the ball down the middle," coach Joe Philbin said after Saturday's game. "We have some big targets. Egnew's a big man. Dion Sims is a big man. Clay has caught the ball down the middle before. We'll adjust."
Keller was carted off the field after being tackled by rookie safety D.J. Swearinger on an incompletion. Swearinger hit Keller in the knee with his helmet, a legal play, and said he went low because he risked a fine by tackling high.
Swearinger sent a tweet on behalf of the tight end, although he referred to him by the wrong name.
"Everybody pray 4 justin keller," Swearinger tweeted. "I pray you have a speedy recovery bro...and kill it when you get back.... DEFINITELY wasnt intentional."
Keller had hoped to parlay a big season in 2013 into a multiyear deal beginning next year. He has 17 career touchdown receptions but played in only eight games in 2012 because of hamstring and ankle injuries.
"Praying my man Dustin Keller 81 has a speedy recovery," Clay tweeted. "Great ball player but even better person."