The two highly-rated young players joined from their respective Premier League clubs earlier in the week and will likely be thrown straight into the first team.
Brewster’s arrival was announced on Tuesday. The England Under-21 international has a Champions League winners medal with Liverpool last season but has struggled for first-team opportunities with Jurgen Klopp’s team.
19-year-old defender Guehi only sealed his move from Stamford Bridge on Friday but he’s eligible to slot straight into Swansea’s backline for Sunday’s South Wales derby.
“I know this is a big chance and a step up for me into the men’s game, I am really excited to be here,” he told the Swansea website.
“The gaffer is a big part of the reason for coming here having worked before with him at England Under-17 level.
“It was huge for me. You are coming to a place where you are unfamiliar with your surroundings, so to have that familiar face really helps in terms of getting down to work.”
Swansea travel to Cardiff on Sunday sitting sixth in the Championship table in good form, with three wins from their last five games.
Their local rivals are six places behind them in the table, but could move within a point of them with a win on home soil.
Brewster replaces Sam Surridge in the Swans’ squad, the striker having been recalled by parent club Bournemouth, while there should be an immediate opportunity for Guehi with defender Mike van der Hoorn facing a race to be fit and Joe Rodon remaining sidelined.
“I wouldn’t bring them here if I did not think they were capable of playing in any game in the Championship,” said the Swansea manager in his pre-match press conference.
“I know Sunday is different, but that’s part and parcel of a Championship or Football League experience.
“Each game has a different context to it, obviously the next one is a big one but if I did not think players could handle it, I would not bring them here.”