Manchester City and Manchester United both moved a step closer to the last 16 of the Europa League with wins on Thursday night but Stoke are in trouble after losing 1-0 at home to Valencia.
Sergio Aguero came off the bench to hand City a 2-1 advantage in their tie against Porto.
The Argentinian struck from close range with six minutes remaining as City came from behind to condemn the Portuguese champions to a rare home defeat at the Estadio do Dragao.
The hosts took a 27th-minute lead in the first leg of the round of 32 clash through Silvestre Varela but an Alvaro Pereira own goal gifted City an equaliser.
Goals in the second half from Ashley Young and Javier Hernandez allowed United to claim a 2-0 victory at Ajax.
Although Ajax were dominant in the first half, the visitors roused themselves after the break.
Young broke the deadlock just before the hour and Hernandez finished off a flowing move to put United in control ahead of next Thursday's second leg at Old Trafford.
Stoke will have to win in the Mestalla next week if their European adventure is to continue after Valencia won at the Britannia Stadium.
Mehmet Topal netted the only goal of the game in the 36th minute with a stunning 30-yard strike that gave Asmir Begovic no chance.
Sofiane Feghouli also hit the post in the second half while Stoke, who have lost their last four Barclays Premier League games, were limited to half-chances.
