Since you have the merging field at the end of the string, wouldn't be easier to append it rather than replace it? You can create a method that will append strings:

public String mergeString(sObject recordObject, String eixstingString)
{
    // Some sObjects like Contact and Lead have FirstName and others have Name field
    return eixstingString + (recordObject.get('Name') == null ? (String)recordObject.get('FirstName') : (String)recordObject.get('Name'));
}

Then you can call it like this:

String sTemp = 'Name belongs to ';
Account oAccount = [select Id, Name FROM Account LIMIT 1];
String mergedString = mergeString(oAccount, sTemp); 

If you had the placeholder in the middle of the string:

String sTemp = 'Name belongs to {recordName} and there is more static text here';

Then you could use the replace method:

public String mergeString(sObject recordObject, String eixstingString)
{
    // Some sObjects like Contact and Lead have FirstName and others have Name field
    return eixstingString.replace('{recordName}', (recordObject.get('Name') == null ? (String)recordObject.get('FirstName') : (String)recordObject.get('Name')));
}

Since you have the merging field at the end of the string, wouldn't be easier to append it rather than replace it? You can create a method that will append strings:

public String mergeString(sObject recordObject, String eixstingString)
{
    // Some sObjects like Contact and Lead have FirstName and others have Name field
    return eixstingString + (recordObject.get('Name') == null ? (String)recordObject.get('FirstName') : (String)recordObject.get('Name'));
}

Then you can call it like this:

String sTemp = 'Name belongs to ';
Account oAccount = [select Id, Name FROM Account LIMIT 1];
String mergedString = mergeString(oAccount, sTemp); 

If you had the placeholder in the middle of the string:

String sTemp = 'Name belongs to {recordName} and there is more static text here';

Then you could use the replace method:

public String mergeString(sObject recordObject, String eixstingString)
{
    // Some sObjects like Contact and Lead have FirstName and others have Name field
    return eixstingString.replace('{recordName}', (recordObject.get('Name') == null ? (String)recordObject.get('FirstName') : (String)recordObject.get('Name')));
}

UPDATE: Based on your comments below it looks like you're trying to dynamically replace a placeholder with the field specified there. I think this final method should get you going:

public String mergeString(sObject recordObject, String eixstingString)
{
    String templateString = eixstingString.substring(eixstingString.indexOf('{'), eixstingString.indexOf('}') + 1); // results in {Account.AccountNumber}
    String fieldName = eixstingString.substring(eixstingString.indexOf('.') + 1, eixstingString.indexOf('}')); // results in "AccountNumber"
    return eixstingString.replace(templateString, (String)recordObject.get(fieldName));
}

Keep in mind that this will not work if you have multiple ., { or } characters in your template. Also you must query the field that is specified in the template.