synchronized block – lock more than one object

A trivial solution would be

synchronized(player) {
    synchronized(field) {
        // code
    }
}

However, make sure that you always lock the resources in the same order to avoid deadlocks.

Note that in practice, the bottleneck is the field, so a single lock on the field (or on a dedicated, common lock object, as @ripper234 rightly pointed out) may suffice (unless you are concurrently manipulating players in other, conflicting ways).