For me there are only 2 real options here.......
1) Sack him now, right now, and get someone new in ahead of the second leg against Dortmund (as it's the only game that really matters between now and the end of the season unless we qualify for the next round)
2) Stick with him till the end of the season and make a decision then. As once we are out of the Champions League, there will be little to no point changing the manager (unless the manager is seriously unpopular inside the dressing room). We won't make top 4 and we aren't going to be relegated, so outside of those two (very unlikely) possibilities, results don't really matter that much and the pressure is largely off.