A number of the league's top recent recruits – including Steven Gerrard, Frank Lampard, Andrea Pirlo and David Villa – have been tipped to agree temporary deals in the coming months.
But Gerrard and Lampard have already ruled out loan switches, while Villa has said he does not expect New York City to sanction such a transfer.
Donovan, who himself had a spell with Bayern Munich and two moves to Everton while playing for LA Galaxy, believes the players would be wise to fully focus on MLS in the current climate.
"You have to take it more seriously now," he said after playing in David Beckham's UNICEF charity match at Old Trafford.
"There is more money involved, more attention paid to it and the fans actually genuinely care more now.
"There is more pressure and you have to really take it more seriously than you probably did 10 years ago."
Donovan, who won the MLS Cup four times with the Galaxy, feels it is positive that a number of players are now ruling out loan moves, in something of a shift in attitude from the likes of Beckham and Thierry Henry.
However, he acknowledges the league still has a long way to go to match the top divisions in Europe and wants there to be more focus on developing world-class American talent as well as attracting big names from abroad.
Donovan continued: "That is a good sign [that players are ruling out moves]. We are not naive and we know we have a long way to go but we are trying to get there.
"We have foreign players who care and want to come and make a difference, which is great.
"But we are also trying to develop our own players and produce players who can play on the world level."