Yes - but beware of negative reactions
The easiest way is to look at what the fan confidence section is saying about players - if they think the player is playing like a muppet, then it's a fairly safe bet that saying something won't trigger a truly negative reaction. Saying something negative like that is useful for certain players at certain times - as with positive comments - so you need to learn how your players respond and when they'll give that response... time-consuming, but then there is the possibility of putting notes on the player's profile
Eventually, you'll secondguess the circumstances and be able to head things off before the fans even notice...
As Anton says, if you've truly had enough of bad form, or perhaps it's a young player who has been overplayed, saying that you're resting them and then doing it for the time period you have said publicly will encourage better performances.(And with some players, usually the young ones, it also seems to trigger the CA increases becoming apparant in increased stats. Just a random edited in thought which I need to think about some more.)
Also take a look at the pre-match teamtalk options for individual players who have performed badly - "I expect better" is an option which can give results, providing that it doesn't add too much pressure onto a mentally weak player.
If interest from another club is unsettling the player, it will show in his personal profile or in the AM feedback (and you've just given me the link between morale and motivation I was looking for - so huge, huge, huge thank yous! What you said has triggered something because I have a player who is going through exactly the same nerves before the match without it hitting his personal profile but sufficiently bad for it to be picked up pre-match by the AM. Thank you
). My usual trick is to increase the player's value by 10 times when a wnt tag appears (obviously this might not always be possible by quite so much - but ramp it up as much as you can without risking the 'player thinks he's overpriced' worry). After a day or so the interest fades away (and if not, how about doubling or tripling that value
) and the player's nerves will settle again, eventually.