Derby appeared to be closing in on a top-two spot a few weeks back, but they have lost both their front-men to injury and Steve McClaren now has no natural senior striker available to him.
It means that the Rams' goalscoring threat has decreased and their form of late has seen them slide down the table.
Although Derby are still in the thick of the race for an automatic promotion place, Lee thinks they will fall short and doubts that they will have enough quality available to win the play-offs.
"My old club have lost their strikers to injury at the wrong time," Lee said. "Chris Martin has always done very well for them, he’s their focal point, and Darren Bent was a great signing by Steve McClaren. I just think when you lose your strikers, you’re always going to struggle. If they can’t get them fit, then it will affect them in the play-offs too."
If Derby do miss out on promotion, it will be a cruel way for their season to end as they have looked the stand out team in the Championship for so long.
With Bent and Martin fit, the Rams were all set to make it into the Premier League, but there are serious doubts over their promotion credentials now.
Derby have not won any of their last five fixtures and are five points adrift of table-topping Watford.
Indeed, it may be that staying in the play-off spots is starting to become a bigger concern than making the automatic promotion places. The Rams are currently only three points clear of seventh placed Ipswich Town, who have shown great consistency throughout the season, and they will need to get back to winning ways soon if they are to remain in the top-six.