The 28-year-old, who made 29 appearances for the Tangerines last season, has one year remaining on his current contract at Tannadice.
However, he is unlikely to be involved in the season opener against Partick Thistle next month.
"I have told Richie it's going to be difficult for him to get into the team," McNamara told the Dundee Courier.
"I could keep him as cover but that's not fair because he needs to be playing football.
"There is nothing happening yet but I am hoping we get something sorted with him soon. We are well covered in midfield and have youngsters coming through."
United are currently on their pre-season tour of Germany, which began on Sunday with a 4-1 defeat to Union Berlin.