They've signed him up because they think romantically and see signs everywhere.
It's exactly 20 years since his last gasp goal won them the European Cup, and he's the manager. It's a sign
They won that cup on 'that night in Barcelona' and they play Barcelona next in the same competition. It's a sign.
When Chelsea sacked a manager and gave the job to one of their old boys with no experience, he took them to a CL victory. It's a sign
When they used to struggle in games Solksjaer used to come on as a late substitute and win it for them. Now he's come on as a late substitute manager to save their season. It's a sign.
And no doubt there are others, too.
They believe in fate, and think it's fate that he is here right now, when they need him, as always, and that the same fate will take them to glory again.
Many clubs have tried this romantic, fateful, Messiah-based approach before, and few, if any, have succeeded. Winning things at the top of the game needs a clear-vision and a hard-headed approach, not something akin to astrology.