The reason for Thompson's promotion is quite simple: the Lions were running out of healthy tight ends. Joseph Fauria has been out for nearly a month with an ankle injury, and Eric Ebron went down in practice this week with a hamstring injury. Fauria has already been ruled out for Sunday's game against the New Orleans Saints, and Ebron is listed as doubtful.
With Thompson being the only tight end on the practice squad, it was a no-brainer to promote him. He joined the Lions back in April, and he emerged as their fourth tight end in training camp. There was only room for three tight ends on the Lions' initial 53-man roster, but injuries have obviously changed the outlook at that position.
With the Lions needing room for Thompson, Lewis became the odd man out. He's been dealing with a quadricep injury since Week 2, and the team decided to just go ahead and end his season by putting him on IR. Although this leaves the Lions down a man at linebacker, they're going to get Kyle Van Noy back after their bye. Chances are Thompson will return to the practice squad at that point to make room for Van Noy, but the Lions don't have to worry about that until Week 10.