Arsenal are closing in on a £75m deal for the Brazilian midfielder, according to BBC Sport, after a breakthrough in talks this week saw Guimarães depart Newcastle United's pre-season training camp in Spain to bid an emotional farewell to team-mates and staff.
Jaissle, appointed as Eddie Howe's successor this summer, revealed he had only a brief exchange with the 28-year-old before his exit was confirmed.
"I met him quickly, but the decision was already made," Jaissle told reporters in his first media appearance since taking charge, describing it as a warm parting with a player he called a fantastic footballer and a great person, before adding that the club now needed to focus on the challenge ahead rather than dwell on his departure.
Guimarães becomes the fourth key player to leave Newcastle in under a year, following Anthony Gordon's move to Barcelona and Sandro Tonali's switch to Tottenham Hotspur earlier in the window, as well as Alexander Isak's transfer to Liverpool last summer.
Club great Alan Shearer struck a cautious tone in a BBC Sport column, warning of "very challenging times ahead" for Newcastle this season, though he also sent the new head coach his best wishes.
Rather than lament the exodus, Jaissle is framing the rebuild as an opportunity to shape a younger, hungrier squad, acknowledging the scale of the "transitions" under way after the club lost several of its "legends" this summer. He said he wants his players focused on "what we can influence" rather than dwelling on departures.
Newcastle have shifted their transfer strategy this summer towards younger, continental talent, with goalkeeper Lukas Hornicek, 24, the club's oldest new arrival by some distance.
Around £146m has been committed so far on Hornicek, fellow goalkeeper Ewen Jaouen, midfielders Aladji Bamba and Sean Steur, and winger Bazoumana Toure - none of whom have previous Premier League experience.
Jaissle himself arrives having never worked in England, but with trophy-laden spells at Al-Ahli in Saudi Arabia and RB Salzburg in Austria behind him, saying it had long been a dream to test himself in the Premier League.
Newcastle open their season against Liverpool at St James' Park on 23 August.