The Vikings worked out the former Texas A&M Aggies signal caller on Friday and Zimmer admitted that he took the opportunity to find out more about the 21-year-old following several off-the-field incidents.
Zimmer told 104.9 The Horn: "We asked him all kinds of questions. There are some flags that come up. All of the things that happened out in Los Angeles, the commercials and all that stuff.
"The position of quarterback in the NFL is such an important position and the reason these guys need to be a totally football-minded guy is the pressure of the position and being the face of an NFL team and doing everything right.
"That's the thing you want to know about him, will he be into work early every single day? Will be the last to leave? Will he be the guy that is working the hardest to get better?"
Manziel threw for 4,114 yards along with 37 touchdowns to 13 interceptions in the 2013 season for the Aggies.