Arsenal have been linked with a move for goalkeeper Petr Cech, and as much as he would improve their options - the flip side of the coin is obvious.
League leaders Chelsea could be tempted into a mistake which could land up costing them the league.
Yes, Thibaut Courtois is one of the best goalkeepers in the world, but is their third choice stopper Mark Schwarzer up to the challenge of seeing out a title bid?
This would become the key question if Belgian international Courtois picked up an injury after the January transfer window, and Chelsea had sold Cech.
Mourinho is probably unwilling to sell to Arsenal, but their interest could also spark offers from other sides and turn the Czech Republic international's head away from Stamford Bridge, where he is now just a reserve.
Schwarzer is a competent stopper, but at 42-years-old his best days are long gone, and even at his peak he was not in the same class as Courtois or Cech.
Last season when he was called upon against Sunderland he made a bad gaffe, which contributed to what turned out to be a crucial loss.
Chelsea have a clear points advantage already in the title race, but something like a goalkeeper injury could throw them off course. If they have Cech, then they are well covered, but should Arsenal - or another side- persuade Chelsea to sell, it could be their undoing.