The added pressure for him to peform from the get go rather than being eased in a new league and country and adapting to Pep's methods. The player would probably want to join the club in the following summer where he would have the time to really get used to been in a new environment.Why does the club's form matter? For example Chelsea made a huge amount of signings when they were lingering mid table and couldn't buy a win.
Every move comes with pressure no matter when it happens. If anyone were to sign in January it would give them an extra few months to bed in given that its highly likely we are out of the race for the league.