The 22-year-old started United's opening two pre-season friendlies under Michael Carrick following his return to Old Trafford, but was left on the bench for Saturday's 2-1 win over Atletico Madrid, a decision read by many as a hint at his impending departure.
Vitek is now on the verge of completing a permanent move to the Riverside, with multiple reports indicating United will insert both a sell-on clause and a buy-back option into the deal. No fee has been reported. The Czech international impressed on loan at Bristol City last season, keeping 12 clean sheets in 41 Championship appearances and sweeping the club's end-of-season awards, sparking wider interest that also included Hull City and clubs in Scotland and Germany.
Vitek made it clear at the start of pre-season that he was ready to leave United in search of regular minutes, telling reporters: "I feel great and ready to go again somewhere to be, hopefully, number one, so we will see. We will see what is going to happen.
"At the moment, I am still here, but we will see. I hope so. I hope there will be some interest. I think you will know when things are more concrete."
With Senne Lammens established as United's undisputed first choice and Karl Darlow arriving on a free transfer from Leeds United as back-up, opportunities for Vitek at Old Trafford were always going to be limited. Middlesbrough, who narrowly missed out on promotion to the Premier League last season after defeat by Hull City in the play-off final, have identified a senior goalkeeper as a priority this summer and see Vitek as an upgrade on their current options behind first-choice Sol Brynn.
Vitek joined Manchester United from Czech side Sigma Olomouc as a 16-year-old and has never made a first-team appearance for the club, instead building experience through loan spells, including at Accrington Stanley and in Austria with Blau-Weiss Linz, before his breakthrough at Bristol City.
He is understood to be keen on Middlesbrough specifically given the promise of regular Championship football, having already stated a preference for a permanent switch over a further loan. Kim Hellberg's Boro side will target another promotion push next season, and staff see the 22-year-old as a clear upgrade in a position identified as needing strengthening this summer.