The Patriots may very well sign free agent QB Johnny Manziel, but don't expect it to happen anytime soon. The Pats are much more likely to wait until after the NFL Draft to sign Manziel, as if a quarterback they like falls to them in the draft and they deem that QB to have good value - they'll roll with that player and current backup Brian Hoyer in camp.
When the Patriots took a flier on free agent Tim Tebow in 2013 - they did not sign him until June of that year. Tebow signed a two-year deal that included no guaranteed money at the time and was released right before the start of that year's regular season. Ryan Mallett beat out Tebow as Tom Brady's backup that summer.
Hurting Manziel's chances of signing with the Pats - let alone making the team - is that Hoyer is still in place. Part of the Patriots' rationale in trading Jimmy Garoppolo last fall was that they were able to get a backup quarterback used to their system in return in Hoyer (Hoyer was technically released by the 49ers).
If the Patriots cannot find a potential successor for Brady in the draft this year or next, Hoyer could very well be a bridge between eras at QB for the organization. Bill Belichick and offensive coordinator Josh McDaniels went 11-5 with Matt Cassel in 2008, so there is a train of thought in Foxboro that they could still have success without the greatest quarterback of all-time. In 2001 when the Patriots won their first Super Bowl, Belichick rarely had Brady throwing the ball more than 35 times in a game (in the Super Bowl win over St. Louis he threw the ball just 27 times).
The Patriots believe they could more than survive with Hoyer under center.
"He's from Ohio, so I have a fondness for those guys," McDaniels said last year when the Pats re-acquired Hoyer. "You know, he's a football kid. He's all about football and loves the game, cares about playing quarterback the right way, smart guy, works hard, good teammate, so there's a lot to like."
In recent years, the Patriots have mostly elected to keep just two quarterbacks on the roster - again hurting the chances that Manziel is a part of the 2018 Pats. Since 2009 (when the Pats originally signed the undrafted Hoyer), the Pats have carried three QBs on their roster in a season just twice - and one of those years came when Brady was suspended the first four games for his alleged involvement in Deflategate.