According to the Daily Express, Atletico Madrid have targeted Carlos Vela as their replacement for the Chelsea bound Costa. But they want to get a deal in place for the Mexican international before putting the rubber stamp on Costa's Stamford Bridge move.
This sounds simple enough with Atletico ready to pay £8m to Real Sociedad for the former-Arsenal forward.
But Arsenal have a buy-back clause inserted into Vela's Real Sociedad contract, and they are tempted to trigger it to bring him back to the Emirates Stadium.
That would be a disaster for Chelsea though, as Atletico would be forced to put the Costa deal on hold. They've already seen David Villa depart for New York City FC, and the La Liga champions cannot afford to lose another forward without a replacement secured.
Arsenal can reportedly re-sign Vela for just £3.25m, and Arsene Wenger is weighing up whether it will be worth it considering Vela would prefer to stay in Spain.
Costa has passed his medical with Chelsea, and he's expected to be Jose Mourinho's big summer capture heading into the new season.